fix: show toasts on every screen, including the Unified Inbox and contacts

Only the tab screens and the viewer had a toast host, so a viewer
action that failed after going back to the Unified Inbox or a contact
(both stack screens of their own) raised a toast nobody saw.

Every stack screen now gets a host through the navigator's
`screenLayout`, and only the focused screen's host renders, so the
screen kept live under the top one never shows the same toast a second
time. The tab screens' and the viewer's own hosts are gone.

App.tsx

@@ -2,7 +2,9 @@ import React from 'react';
 import { ActivityIndicator, AppState, Linking, StyleSheet, Text, View } from 'react-native';
 import { StatusBar } from 'expo-status-bar';
 import { useColorScheme } from 'react-native';
-import { DarkTheme, DefaultTheme, NavigationContainer, createNavigationContainerRef } from '@react-navigation/native';
+import {
+  DarkTheme, DefaultTheme, NavigationContainer, createNavigationContainerRef, useIsFocused,
+} from '@react-navigation/native';
 import { createBottomTabNavigator } from '@react-navigation/bottom-tabs';
 import { createNativeStackNavigator, type NativeStackScreenProps } from '@react-navigation/native-stack';
 import { Mail, Calendar, BookUser, HardDrive, Settings } from 'lucide-react-native';
@@ -147,6 +149,24 @@ async function openDeepLink(link: DeepLink): Promise<void> {
   });
 }
 
+// Toasts show on whichever stack screen is up, so a failure that lands after
+// leaving the screen it came from (a viewer action finishing on the Unified
+// Inbox or a contact) is still seen. Every screen carries a host, and only
+// the focused one renders, so a toast never shows twice (the screen below
+// the top one is kept live).
+function FocusedToastHost() {
+  return useIsFocused() ? <ToastHost /> : null;
+}
+
+function withToastHost({ children }: { children: React.ReactElement }) {
+  return (
+    <>
+      {children}
+      <FocusedToastHost />
+    </>
+  );
+}
+
 function LoadingScreen({ message }: { message: string }) {
   const c = useColors();
   return (
@@ -175,7 +195,6 @@ function MainTabsNavigator({ navigation }: NativeStackScreenProps<RootStackParam
       <UpdateBanner />
       <OfflineCacheBanner />
       <PushOnboardingPrompt />
-      <ToastHost />
       <AppIconBadge />
     <Tab.Navigator
       screenOptions={{
@@ -706,7 +725,7 @@ export default function App() {
       <StatusBar style={statusBarStyle} />
       {/* On Fabric, native-stack keeps the screen right below the top live and
           freezes the ones further down. */}
-      <Stack.Navigator screenOptions={{ headerShown: false, freezeOnBlur: true }}>
+      <Stack.Navigator screenOptions={{ headerShown: false, freezeOnBlur: true }} screenLayout={withToastHost}>
         <Stack.Screen name="MainTabs" component={MainTabsNavigator} />
         <Stack.Screen name="EmailThread" component={EmailThreadScreen} />
         <Stack.Screen name="EmailSource" component={EmailSourceScreen} />

src/components/ToastHost.tsx

@@ -9,9 +9,10 @@ import { useToastStore, type Toast } from '../stores/toast-store';
 import { useLocaleStore } from '../stores/locale-store';
 
 /**
- * Renders the toast queue above the tab bar. One host is mounted in App.tsx;
- * the email undo snackbar keeps its own component because it is bound to
- * the email store's pending-undo entry.
+ * Renders the toast queue above the tab bar. App.tsx gives every stack screen
+ * one and only the focused screen's renders; the email undo snackbar keeps
+ * its own component because it is bound to the email store's pending-undo
+ * entry.
  */
 export function ToastHost(): React.ReactElement | null {
   const toasts = useToastStore((s) => s.toasts);

src/screens/EmailThreadScreen.tsx

@@ -18,7 +18,6 @@ import { MoveSheet } from '../components/MoveSheet';
 import { ThreadMessageCard, ThreadCardPlaceholder } from '../components/email/ThreadMessageCard';
 import { QuickReplyBox } from '../components/email/QuickReplyBox';
 import { AddressActionSheet } from '../components/email/AddressActionSheet';
-import { ToastHost } from '../components/ToastHost';
 import { useEmailStore } from '../stores/email-store';
 import { toast } from '../stores/toast-store';
 import {
@@ -895,9 +894,6 @@ function EmailViewer({ route, navigation }: Props) {
       />
 
       <AddressActionSheet address={addressSheet} onClose={() => setAddressSheet(null)} />
-
-      {/* The list's host sits under this screen; failures in here need their own. */}
-      <ToastHost />
     </SafeAreaView>
   );
 }