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/page.tsx

@@ -4,6 +4,7 @@ import { useEffect, useState } from 'react';
 import { AlertTriangle } from 'lucide-react';
 import { SettingsSection, SettingItem, ToggleSwitch } from '@/components/settings/settings-section';
 import type { AuditEntry } from '@/lib/admin/types';
+import { apiFetch } from '@/lib/browser-navigation';
 
 interface AdminStatus {
   enabled: boolean;
@@ -38,13 +39,13 @@ export default function AdminDashboardPage() {
 
   async function fetchDashboardData() {
     const [statusRes, auditRes, configRes, adminConfigRes, pluginRes, themeRes, policyRes] = await Promise.all([
-      fetch('/api/admin/auth'),
-      fetch('/api/admin/audit?limit=10'),
-      fetch('/api/config'),
-      fetch('/api/admin/config'),
-      fetch('/api/admin/plugins').catch(() => null),
-      fetch('/api/admin/themes').catch(() => null),
-      fetch('/api/admin/policy').catch(() => null),
+      apiFetch('/api/admin/auth'),
+      apiFetch('/api/admin/audit?limit=10'),
+      apiFetch('/api/config'),
+      apiFetch('/api/admin/config'),
+      apiFetch('/api/admin/plugins').catch(() => null),
+      apiFetch('/api/admin/themes').catch(() => null),
+      apiFetch('/api/admin/policy').catch(() => null),
     ]);
 
     if (statusRes.ok) setStatus(await statusRes.json());
@@ -75,7 +76,7 @@ export default function AdminDashboardPage() {
 
     if (configData?.jmapServerUrl) {
       try {
-        const jmapRes = await fetch('/api/config');
+        const jmapRes = await apiFetch('/api/config');
         setJmapHealth(jmapRes.ok ? 'ok' : 'error');
       } catch {
         setJmapHealth('error');