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/theme-store.ts

@@ -6,6 +6,7 @@ import { injectThemeCSS, removeThemeCSS, sanitizeThemeCSS } from '@/lib/theme-lo
 import { extractTheme } from '@/lib/plugin-validator';
 import { BUILTIN_THEMES } from '@/lib/builtin-themes';
 import { usePolicyStore } from '@/stores/policy-store';
+import { apiFetch } from '@/lib/browser-navigation';
 
 type Theme = 'light' | 'dark' | 'system';
 
@@ -299,7 +300,7 @@ export const useThemeStore = create<ThemeState>()(
 
         themeSyncPromise = (async () => {
           try {
-            const res = await fetch('/api/plugins');
+            const res = await apiFetch('/api/plugins');
             if (!res.ok) return;
 
             const data: { themes: ServerThemeInfo[] } = await res.json();
@@ -480,7 +481,7 @@ function dedupeInstalledThemes(themes: InstalledTheme[]): InstalledTheme[] {
 
 async function downloadThemeCSS(themeId: string): Promise<string | null> {
   try {
-    const res = await fetch(`/api/admin/themes/${encodeURIComponent(themeId)}/css`);
+    const res = await apiFetch(`/api/admin/themes/${encodeURIComponent(themeId)}/css`);
     if (!res.ok) return null;
     return await res.text();
   } catch {