diff --git a/app/(main)/[locale]/calendar/[[...segments]]/page.tsx b/app/(main)/[locale]/calendar/[[...segments]]/page.tsx
index 7ac1875fd..3419c08b2 100644
--- a/app/(main)/[locale]/calendar/[[...segments]]/page.tsx
+++ b/app/(main)/[locale]/calendar/[[...segments]]/page.tsx
@@ -1,5 +1,6 @@
 import { CalendarApp } from "@/components/calendar/calendar-app";
 import { generateLiteSegmentParams } from "@/lib/lite-static-params";
+import { AppErrorBoundary } from "@/components/error/app-error-boundary";
 
 // Calendar permalinks (#733):
 //   /calendar                        the stored view, today
@@ -17,5 +18,9 @@ export default async function CalendarRoute({
   params: Promise<{ segments?: string[] }>;
 }) {
   const { segments } = await params;
-  return <CalendarApp linkSegments={segments ?? []} />;
+  return (
+    <AppErrorBoundary>
+      <CalendarApp linkSegments={segments ?? []} />
+    </AppErrorBoundary>
+  );
 }
diff --git a/app/(main)/[locale]/contacts/[[...segments]]/page.tsx b/app/(main)/[locale]/contacts/[[...segments]]/page.tsx
index 7c18db300..da9f8ae44 100644
--- a/app/(main)/[locale]/contacts/[[...segments]]/page.tsx
+++ b/app/(main)/[locale]/contacts/[[...segments]]/page.tsx
@@ -1,5 +1,6 @@
 import { ContactsApp } from "@/components/contacts/contacts-app";
 import { generateLiteSegmentParams } from "@/lib/lite-static-params";
+import { AppErrorBoundary } from "@/components/error/app-error-boundary";
 
 // Contact permalinks (#733): /contacts/<contactId>[/edit], plus /contacts/new
 // for the "add this sender" flow. The older query form (?contactId=, ?addEmail=)
@@ -15,5 +16,9 @@ export default async function ContactsRoute({
   params: Promise<{ segments?: string[] }>;
 }) {
   const { segments } = await params;
-  return <ContactsApp linkSegments={segments ?? []} />;
+  return (
+    <AppErrorBoundary>
+      <ContactsApp linkSegments={segments ?? []} />
+    </AppErrorBoundary>
+  );
 }
diff --git a/app/(main)/[locale]/error.tsx b/app/(main)/[locale]/error.tsx
index e8a0caeac..f25e2ad46 100644
--- a/app/(main)/[locale]/error.tsx
+++ b/app/(main)/[locale]/error.tsx
@@ -2,9 +2,7 @@
 
 import { useEffect } from "react";
 import { useTranslations } from "next-intl";
-import { AlertCircle, RefreshCw, Home } from "@/components/icons";
-import { Button } from "@/components/ui/button";
-import { useRouter } from "@/i18n/navigation";
+import { ErrorState } from "@/components/error/error-fallbacks";
 
 /**
  * Route-level error boundary for locale pages.
@@ -18,35 +16,19 @@ export default function LocaleError({
   reset: () => void;
 }) {
   const t = useTranslations("errors");
-  const router = useRouter();
 
   useEffect(() => {
     console.error("Route error:", error);
   }, [error]);
 
   return (
-    <div className="min-h-screen flex items-center justify-center bg-background">
-      <div className="text-center max-w-md px-4">
-        <div className="w-16 h-16 mx-auto mb-6 rounded-full bg-red-100 dark:bg-red-900/20 flex items-center justify-center">
-          <AlertCircle className="w-8 h-8 text-red-600 dark:text-red-400" />
-        </div>
-        <h2 className="text-xl font-semibold text-foreground mb-2">
-          {t("page_error_title")}
-        </h2>
-        <p className="text-muted-foreground mb-6">
-          {t("page_error_description")}
-        </p>
-        <div className="flex gap-3 justify-center">
-          <Button variant="outline" onClick={() => router.push('/')}>
-            <Home className="w-4 h-4 me-2" />
-            {t("go_home")}
-          </Button>
-          <Button onClick={reset}>
-            <RefreshCw className="w-4 h-4 me-2" />
-            {t("try_again")}
-          </Button>
-        </div>
-      </div>
-    </div>
+    <ErrorState
+      className="min-h-screen"
+      title={t("page_error_title")}
+      description={t("page_error_description")}
+      onRetry={reset}
+      retryLabel={t("try_again")}
+      homeLabel={t("go_home")}
+    />
   );
 }
diff --git a/app/(main)/[locale]/files/[[...segments]]/page.tsx b/app/(main)/[locale]/files/[[...segments]]/page.tsx
index a9782d6f6..a7a885744 100644
--- a/app/(main)/[locale]/files/[[...segments]]/page.tsx
+++ b/app/(main)/[locale]/files/[[...segments]]/page.tsx
@@ -1,5 +1,6 @@
 import { FilesApp } from "@/components/files/files-app";
 import { generateLiteSegmentParams } from "@/lib/lite-static-params";
+import { AppErrorBoundary } from "@/components/error/app-error-boundary";
 
 // Files permalinks (#733): /files/<folder>/<subfolder>, one path segment per
 // level, with ?preview=<name> to open a file straight into the preview modal.
@@ -14,5 +15,9 @@ export default async function FilesRoute({
   params: Promise<{ segments?: string[] }>;
 }) {
   const { segments } = await params;
-  return <FilesApp linkSegments={segments ?? []} />;
+  return (
+    <AppErrorBoundary>
+      <FilesApp linkSegments={segments ?? []} />
+    </AppErrorBoundary>
+  );
 }
diff --git a/app/(main)/[locale]/settings/[[...segments]]/page.tsx b/app/(main)/[locale]/settings/[[...segments]]/page.tsx
index 8c9b582d5..e4dbc45e8 100644
--- a/app/(main)/[locale]/settings/[[...segments]]/page.tsx
+++ b/app/(main)/[locale]/settings/[[...segments]]/page.tsx
@@ -1,5 +1,6 @@
 import { SettingsApp } from "@/components/settings/settings-app";
 import { generateLiteSegmentParams } from "@/lib/lite-static-params";
+import { AppErrorBoundary } from "@/components/error/app-error-boundary";
 
 // Settings permalinks (#733): /settings/<tabId>. Without a tab the page falls
 // back to the last tab the user had open, as before.
@@ -14,5 +15,9 @@ export default async function SettingsRoute({
   params: Promise<{ segments?: string[] }>;
 }) {
   const { segments } = await params;
-  return <SettingsApp linkSegments={segments ?? []} />;
+  return (
+    <AppErrorBoundary kind="settings">
+      <SettingsApp linkSegments={segments ?? []} />
+    </AppErrorBoundary>
+  );
 }
diff --git a/components/error/__tests__/error-fallbacks.test.tsx b/components/error/__tests__/error-fallbacks.test.tsx
new file mode 100644
index 000000000..da84607e8
--- /dev/null
+++ b/components/error/__tests__/error-fallbacks.test.tsx
@@ -0,0 +1,56 @@
+import React from 'react';
+import { fireEvent, render, screen } from '@testing-library/react';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+// One error fallback everywhere: the same "Try again", a way back to the
+// inbox outside mail, and each app has a boundary of its own that keeps the
+// navigation on screen.
+
+vi.mock('@/components/layout/navigation-rail', () => ({
+  NavigationRail: ({ orientation }: { orientation?: string }) => <nav aria-label={`rail-${orientation ?? 'vertical'}`} />,
+}));
+
+const { AppErrorBoundary } = await import('../app-error-boundary');
+const { ErrorBoundary } = await import('../error-boundary');
+const { EmailListErrorFallback, SidebarErrorFallback, SettingsErrorFallback } = await import('../error-fallbacks');
+
+let broken = true;
+function Flaky() {
+  if (broken) throw new Error('boom');
+  return <p>calendar works</p>;
+}
+
+beforeEach(() => {
+  broken = true;
+  vi.spyOn(console, 'error').mockImplementation(() => {});
+});
+afterEach(() => vi.restoreAllMocks());
+
+describe('app error boundary', () => {
+  it('shows the fallback next to the navigation and recovers on Try again', () => {
+    render(<AppErrorBoundary><Flaky /></AppErrorBoundary>);
+    expect(screen.getByRole('alert')).toHaveTextContent('page_error_title');
+    expect(screen.getByRole('navigation', { name: 'rail-vertical' })).toBeInTheDocument();
+    expect(screen.getByRole('link', { name: 'go_home' })).toBeInTheDocument();
+
+    broken = false;
+    fireEvent.click(screen.getByRole('button', { name: 'try_again' }));
+    expect(screen.getByText('calendar works')).toBeInTheDocument();
+  });
+});
+
+describe('section fallbacks', () => {
+  it.each([
+    ['message list', EmailListErrorFallback],
+    ['folder sidebar', SidebarErrorFallback],
+    ['settings', SettingsErrorFallback],
+  ])('the %s offers the same "Try again"', (_name, fallback) => {
+    render(<ErrorBoundary fallback={fallback}><Flaky /></ErrorBoundary>);
+    expect(screen.getByRole('button', { name: 'try_again' })).toBeInTheDocument();
+  });
+
+  it('inside mail, does not offer to go to the inbox it is already in', () => {
+    render(<ErrorBoundary fallback={EmailListErrorFallback}><Flaky /></ErrorBoundary>);
+    expect(screen.queryByRole('link', { name: 'go_home' })).toBeNull();
+  });
+});
diff --git a/components/error/app-error-boundary.tsx b/components/error/app-error-boundary.tsx
new file mode 100644
index 000000000..5b36c5037
--- /dev/null
+++ b/components/error/app-error-boundary.tsx
@@ -0,0 +1,39 @@
+"use client";
+
+import type { ReactNode } from "react";
+import { NavigationRail } from "@/components/layout/navigation-rail";
+import { useIsMobile } from "@/hooks/use-media-query";
+import { ErrorBoundary, type FallbackProps } from "./error-boundary";
+import { PageErrorFallback, SettingsErrorFallback } from "./error-fallbacks";
+
+function AppErrorLayout({ kind, ...props }: FallbackProps & { kind: "page" | "settings" }) {
+  const isMobile = useIsMobile();
+  const Fallback = kind === "settings" ? SettingsErrorFallback : PageErrorFallback;
+  // The navigation stays, so one app crashing doesn't strand the user in it.
+  return (
+    <div className="flex h-dvh bg-background pt-[env(safe-area-inset-top)]">
+      {!isMobile && (
+        <div className="w-14 bg-secondary flex flex-col flex-shrink-0 border-e border-border">
+          <NavigationRail collapsed />
+        </div>
+      )}
+      <div className="flex flex-col flex-1 min-w-0">
+        <Fallback {...props} />
+        {isMobile && <NavigationRail orientation="horizontal" />}
+      </div>
+    </div>
+  );
+}
+
+/**
+ * Wraps one app (Calendar, Contacts, Files, Settings): a crash in it shows
+ * the shared error fallback next to the navigation, instead of the route
+ * error page in place of everything.
+ */
+export function AppErrorBoundary({ children, kind = "page" }: { children: ReactNode; kind?: "page" | "settings" }) {
+  return (
+    <ErrorBoundary fallback={(props) => <AppErrorLayout kind={kind} {...props} />}>
+      {children}
+    </ErrorBoundary>
+  );
+}
diff --git a/components/error/error-fallbacks.tsx b/components/error/error-fallbacks.tsx
index a76afffb1..d58d68633 100644
--- a/components/error/error-fallbacks.tsx
+++ b/components/error/error-fallbacks.tsx
@@ -1,127 +1,107 @@
 "use client";
 
-import { AlertCircle, RefreshCw, Inbox, Mail, Settings, FolderOpen } from "@/components/icons";
+import { AlertCircle, RefreshCw } from "@/components/icons";
+import type { AppIcon } from "@/components/icons";
 import { Button } from "@/components/ui/button";
+import { getPathPrefix } from "@/lib/browser-navigation";
+import { cn } from "@/lib/utils";
 import type { FallbackProps } from "./error-boundary";
 
+export interface ErrorStateProps {
+  title: string;
+  description?: string;
+  /** Puts the part back together: re-renders it, or reloads the route. */
+  onRetry: () => void;
+  retryLabel: string;
+  /** Offers a way out to the inbox; left out inside mail itself. */
+  homeLabel?: string;
+  icon?: AppIcon;
+  className?: string;
+}
+
 /**
- * Full-page error fallback for route-level errors.
+ * The one error fallback: icon, title, a line of text, "Try again", and "Go
+ * to inbox" where the inbox is somewhere else. Every error boundary draws
+ * this, at whatever size its pane has.
  */
-export function PageErrorFallback({ error: _error, resetError, t }: FallbackProps) {
+export function ErrorState({ title, description, onRetry, retryLabel, homeLabel, icon: Icon = AlertCircle, className }: ErrorStateProps) {
   return (
-    <div className="flex h-screen items-center justify-center bg-background">
-      <div className="text-center max-w-md px-4">
-        <div className="w-16 h-16 mx-auto mb-6 rounded-full bg-red-100 dark:bg-red-900/20 flex items-center justify-center">
-          <AlertCircle className="w-8 h-8 text-red-600 dark:text-red-400" />
-        </div>
-        <h2 className="text-xl font-semibold text-foreground mb-2">
-          {t("page_error_title")}
-        </h2>
-        <p className="text-muted-foreground mb-6">
-          {t("page_error_description")}
-        </p>
-        <Button onClick={resetError}>
+    <div role="alert" className={cn("flex flex-1 h-full flex-col items-center justify-center p-8 text-center bg-background", className)}>
+      <div className="w-12 h-12 mb-4 rounded-full bg-red-100 dark:bg-red-900/20 flex items-center justify-center">
+        <Icon className="w-6 h-6 text-red-600 dark:text-red-400" aria-hidden="true" />
+      </div>
+      <h2 className="text-lg font-semibold text-foreground mb-2">{title}</h2>
+      {description && <p className="text-sm text-muted-foreground mb-6 max-w-md">{description}</p>}
+      <div className={cn("flex flex-wrap gap-3 justify-center", !description && "mt-4")}>
+        {homeLabel && (
+          // A full load: whatever broke starts over.
+          <a
+            href={`${getPathPrefix()}/`}
+            className="inline-flex items-center justify-center h-9 px-3 text-sm font-medium rounded-md border border-input bg-background hover:bg-accent hover:text-accent-foreground transition-colors"
+          >
+            {homeLabel}
+          </a>
+        )}
+        <Button size="sm" onClick={onRetry}>
           <RefreshCw className="w-4 h-4 me-2" />
-          {t("try_again")}
+          {retryLabel}
         </Button>
       </div>
     </div>
   );
 }
 
-/**
- * Sidebar error fallback - matches sidebar width (256px).
- */
+/** Full-page error fallback, also used around each app (AppErrorBoundary). */
+export function PageErrorFallback({ resetError, t }: FallbackProps) {
+  return (
+    <ErrorState
+      title={t("page_error_title")}
+      description={t("page_error_description")}
+      onRetry={resetError}
+      retryLabel={t("try_again")}
+      homeLabel={t("go_home")}
+    />
+  );
+}
+
+/** Mail's folder sidebar (256px wide). */
 export function SidebarErrorFallback({ resetError, t }: FallbackProps) {
   return (
-    <div className="w-64 h-full border-e border-border bg-secondary flex flex-col items-center justify-center p-4">
-      <FolderOpen className="w-10 h-10 text-muted-foreground mb-3" />
-      <p className="text-sm text-muted-foreground text-center mb-4">
-        {t("sidebar_error")}
-      </p>
-      <Button variant="outline" size="sm" onClick={resetError}>
-        <RefreshCw className="w-3 h-3 me-1" />
-        {t("reload")}
-      </Button>
-    </div>
+    <ErrorState className="w-64 flex-none border-e border-border bg-secondary p-4" title={t("sidebar_error")} onRetry={resetError} retryLabel={t("try_again")} />
   );
 }
 
-/**
- * Email list error fallback - matches email list panel width (384px).
- */
+/** Mail's message list. */
 export function EmailListErrorFallback({ resetError, t }: FallbackProps) {
-  return (
-    <div className="w-full h-full bg-background flex flex-col items-center justify-center p-4">
-      <Inbox className="w-12 h-12 text-muted-foreground mb-3" />
-      <p className="text-sm text-muted-foreground text-center mb-4">
-        {t("email_list_error")}
-      </p>
-      <Button variant="outline" size="sm" onClick={resetError}>
-        <RefreshCw className="w-4 h-4 me-2" />
-        {t("reload_emails")}
-      </Button>
-    </div>
-  );
+  return <ErrorState className="w-full p-4" title={t("email_list_error")} onRetry={resetError} retryLabel={t("try_again")} />;
 }
 
-/**
- * Email viewer error fallback - fills remaining space (flex-1).
- */
+/** Mail's reading pane. */
 export function EmailViewerErrorFallback({ resetError, t }: FallbackProps) {
   return (
-    <div className="flex-1 flex flex-col items-center justify-center bg-muted/30 p-8">
-      <div className="w-16 h-16 mx-auto mb-4 rounded-full bg-red-50 dark:bg-red-900/20 flex items-center justify-center">
-        <Mail className="w-8 h-8 text-red-500" />
-      </div>
-      <h3 className="text-lg font-medium text-foreground mb-2">
-        {t("viewer_error_title")}
-      </h3>
-      <p className="text-sm text-muted-foreground text-center mb-6 max-w-md">
-        {t("viewer_error_description")}
-      </p>
-      <Button onClick={resetError}>
-        <RefreshCw className="w-4 h-4 me-2" />
-        {t("try_again")}
-      </Button>
-    </div>
+    <ErrorState
+      title={t("viewer_error_title")}
+      description={t("viewer_error_description")}
+      onRetry={resetError}
+      retryLabel={t("try_again")}
+    />
   );
 }
 
-/**
- * Email composer modal error fallback.
- */
+/** The composer. */
 export function ComposerErrorFallback({ resetError, t }: FallbackProps) {
-  return (
-    <div className="flex flex-col h-full bg-background border rounded-lg items-center justify-center p-8">
-      <AlertCircle className="w-10 h-10 text-warning mb-3" />
-      <p className="text-sm text-muted-foreground text-center mb-4">
-        {t("composer_error")}
-      </p>
-      <Button variant="outline" size="sm" onClick={resetError}>
-        {t("retry")}
-      </Button>
-    </div>
-  );
+  return <ErrorState className="border rounded-lg" title={t("composer_error")} onRetry={resetError} retryLabel={t("try_again")} />;
 }
 
-/**
- * Settings page error fallback.
- */
+/** Settings. */
 export function SettingsErrorFallback({ resetError, t }: FallbackProps) {
   return (
-    <div className="flex-1 flex flex-col items-center justify-center p-8">
-      <Settings className="w-12 h-12 text-muted-foreground mb-4" />
-      <h3 className="text-lg font-medium text-foreground mb-2">
-        {t("settings_error_title")}
-      </h3>
-      <p className="text-sm text-muted-foreground text-center mb-6">
-        {t("settings_error_description")}
-      </p>
-      <Button onClick={resetError}>
-        <RefreshCw className="w-4 h-4 me-2" />
-        {t("reload_settings")}
-      </Button>
-    </div>
+    <ErrorState
+      title={t("settings_error_title")}
+      description={t("settings_error_description")}
+      onRetry={resetError}
+      retryLabel={t("try_again")}
+      homeLabel={t("go_home")}
+    />
   );
 }
diff --git a/components/error/index.ts b/components/error/index.ts
index 2b70083be..fef3b8dcf 100644
--- a/components/error/index.ts
+++ b/components/error/index.ts
@@ -1,6 +1,8 @@
 export { ErrorBoundary } from "./error-boundary";
+export { AppErrorBoundary } from "./app-error-boundary";
 export type { FallbackProps } from "./error-boundary";
 export {
+  ErrorState,
   PageErrorFallback,
   SidebarErrorFallback,
   EmailListErrorFallback,