fix: send the WOPI status and launch requests under the base path

Behind a reverse proxy at a sub-path (NEXT_PUBLIC_BASE_PATH, e.g.
/webmail) both WOPI requests used a bare fetch('/api/wopi/...') and
reached the mail server instead of the webmail. The status probe came
back 404, so office editing always looked unavailable there - and since
#1047 the probe runs for every opened message, logging the 404 each
time. Route both through apiFetch like every other API call, and cover
the sub-path mount with a test.

components/files/__tests__/wopi-base-path.test.tsx

@@ -0,0 +1,39 @@
+import { render, renderHook, waitFor, cleanup } from '@testing-library/react';
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { useWopiStatus } from '@/hooks/use-wopi-status';
+import { WopiEditor } from '@/components/files/wopi-editor';
+
+// Behind a reverse proxy at a sub-path (e.g. /webmail) a bare
+// fetch('/api/...') reaches the mail server instead of the webmail and comes
+// back 404, which left office editing permanently "unavailable" there. Both
+// WOPI requests have to carry the mount prefix.
+
+vi.mock('next-intl', () => ({ useTranslations: () => (key: string) => key }));
+
+let fetchMock: ReturnType<typeof vi.fn>;
+
+beforeEach(() => {
+  window.history.pushState({}, '', '/webmail/de/mail');
+  fetchMock = vi.fn().mockResolvedValue({ ok: false, status: 404, json: async () => ({}) });
+  vi.stubGlobal('fetch', fetchMock);
+});
+
+afterEach(() => {
+  cleanup();
+  vi.unstubAllGlobals();
+  window.history.pushState({}, '', '/');
+});
+
+describe('WOPI requests under a sub-path mount', () => {
+  it('probes the editor status under the mount prefix', async () => {
+    renderHook(() => useWopiStatus(true));
+    await waitFor(() => expect(fetchMock).toHaveBeenCalled());
+    expect(fetchMock.mock.calls[0][0]).toBe('/webmail/api/wopi/status');
+  });
+
+  it('launches the editor under the mount prefix', async () => {
+    render(<WopiEditor target={{ kind: 'file', id: 'f1', name: 'report.docx' }} onClose={() => {}} />);
+    await waitFor(() => expect(fetchMock).toHaveBeenCalled());
+    expect(fetchMock.mock.calls[0][0]).toBe('/webmail/api/wopi/launch');
+  });
+});

components/files/wopi-editor.tsx

@@ -4,6 +4,7 @@ import { useEffect, useRef, useState } from "react";
 import { useTranslations } from "next-intl";
 import { Loader2, X } from "@/components/icons";
 import { getActiveAccountSlotHeaders } from "@/lib/auth/active-account-slot";
+import { apiFetch } from "@/lib/browser-navigation";
 
 /** What the overlay opens: a Files node, or a mail attachment (read-only, #1047). */
 export type WopiDocument =
@@ -55,7 +56,7 @@ export function WopiEditor({ target, accountId, slot, onClose }: WopiEditorProps
       try {
         const slotHeaders: Record<string, string> =
           typeof slot === "number" ? { "X-JMAP-Cookie-Slot": String(slot) } : getActiveAccountSlotHeaders();
-        const res = await fetch("/api/wopi/launch", {
+        const res = await apiFetch("/api/wopi/launch", {
           method: "POST",
           headers: { "Content-Type": "application/json", ...slotHeaders },
           body: launchBody,

hooks/use-wopi-status.ts

@@ -2,6 +2,7 @@
 
 import { useEffect, useState } from "react";
 import { IS_LITE } from "@/lib/lite";
+import { apiFetch } from "@/lib/browser-navigation";
 
 /**
  * Whether a WOPI document editor is configured for this deployment and which
@@ -22,7 +23,7 @@ function fetchWopiStatus(): Promise<WopiStatus> {
   // The WOPI bridge is a server feature; the static build never has an editor.
   if (IS_LITE) return Promise.resolve(DISABLED);
   if (!statusPromise) {
-    statusPromise = fetch("/api/wopi/status")
+    statusPromise = apiFetch("/api/wopi/status")
       .then((res) => (res.ok ? res.json() : DISABLED))
       .then((data) => ({
         enabled: !!data?.enabled,