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).

app/admin/layout.tsx

@@ -27,6 +27,7 @@ import { useThemeStore } from '@/stores/theme-store';
 import { getActiveAccountSlotHeaders } from '@/lib/auth/active-account-slot';
 
 import { useAuthStore } from '@/stores/auth-store';
+import { apiFetch } from '@/lib/browser-navigation';
 
 const NAV_GROUPS = [
   {
@@ -85,7 +86,7 @@ export default function AdminLayout({ children }: { children: React.ReactNode })
   async function checkAuth() {
     try {
       const jmapHeaders = getJmapHeaders();
-      const res = await fetch('/api/admin/auth', { headers: jmapHeaders });
+      const res = await apiFetch('/api/admin/auth', { headers: jmapHeaders });
       const data = await res.json();
 
       const stalwartAdmin = data.stalwartAdmin === true;
@@ -104,7 +105,7 @@ export default function AdminLayout({ children }: { children: React.ReactNode })
 
       // If Stalwart admin but not yet authenticated, auto-login
       if (stalwartAdmin) {
-        const loginRes = await fetch('/api/admin/auth', {
+        const loginRes = await apiFetch('/api/admin/auth', {
           method: 'POST',
           headers: { 'Content-Type': 'application/json', ...jmapHeaders },
           body: JSON.stringify({ stalwartAuth: true }),
@@ -122,7 +123,7 @@ export default function AdminLayout({ children }: { children: React.ReactNode })
   }
 
   async function handleLogout() {
-    await fetch('/api/admin/auth', { method: 'DELETE' });
+    await apiFetch('/api/admin/auth', { method: 'DELETE' });
     router.replace('/admin/login');
   }