feat: apiFetch helper for mount-prefix-aware API calls

Makes every client-side fetch('/api/...') call respect the mount prefix
when Bulwark is served behind a reverse proxy at a sub-path (e.g.
`/webmail`).

### Problem

`getPathPrefix()` (added in 1.4.13 by #XXX / d762b94) already fixes
router navigation and redirect URIs for reverse-proxy deployments.
Client-side `fetch()` calls, though, still target the browser origin:

    await fetch('/api/foo')
    // Browser at /webmail/en/inbox → hits /api/foo (not proxied → 404)

That means the login flow, session establishment, settings save, plugin
loader, calendar import, etc. all break the moment you front Bulwark
with nginx (or any proxy) at a sub-path.

### Fix

Add `apiFetch(input, init)` next to `getPathPrefix()` in
`lib/browser-navigation.ts`. It prepends the mount prefix to any
absolute path at call time:

    await apiFetch('/api/foo')
    // /webmail/en/inbox → /webmail/api/foo
    // /en/inbox         → /api/foo

Same runtime-detection model as `getPathPrefix()` — the built bundle
works at any mount point without rebuilding or env-var config.
Protocol-relative (`//cdn...`) and absolute (`https://...`) URLs pass
through unchanged. Server-side route handlers are untouched (the mount
prefix is a browser-only concept).

### Migration

Mechanical rewrite of every client-side `fetch('/api/...')` call in
hooks/, lib/, stores/, components/, app/ — 99 call sites across
26 files. `route.ts` handlers and other server-only files are skipped.

### Compat

- No behaviour change when mounted at `/` (the common case): an empty
  prefix + raw path is identical to raw path.
- No new config knobs, env vars, or build flags.
- Supersedes PR #181 (which required a build-time `NEXT_PUBLIC_BASE_PATH`)
  — will close #181 after this lands.

### Testing

Should run the existing suite; smoke-tested by Jabali Panel which
reverse-proxies Bulwark at `/webmail/` (https://github.com/shukiv/jabali-panel).

stores/settings-store.ts

@@ -3,6 +3,7 @@ import { persist } from 'zustand/middleware';
 import { useThemeStore } from './theme-store';
 import { useLocaleStore } from './locale-store';
 import type { NotificationSoundChoice } from '@/lib/notification-sound';
+import { apiFetch } from '@/lib/browser-navigation';
 
 // Use console directly to avoid circular dependency with lib/debug.ts
 // (debug.ts imports useSettingsStore for debugMode check)
@@ -611,7 +612,7 @@ export const useSettingsStore = create<SettingsState>()(
       loadFromServer: async (username: string, serverUrl: string) => {
         try {
           syncLog('Loading settings from server for', username);
-          const res = await fetch('/api/settings', {
+          const res = await apiFetch('/api/settings', {
             headers: {
               'x-settings-username': username,
               'x-settings-server': serverUrl,
@@ -747,7 +748,7 @@ if (typeof window !== 'undefined') {
   const syncToServer = async (retries = 1): Promise<void> => {
     const settings = JSON.parse(useSettingsStore.getState().exportSettings());
     syncLog('Syncing settings to server...');
-    const res = await fetch('/api/settings', {
+    const res = await apiFetch('/api/settings', {
       method: 'POST',
       headers: { 'Content-Type': 'application/json' },
       body: JSON.stringify({ username: syncUsername, serverUrl: syncServerUrl, settings }),