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).

lib/browser-navigation.ts

@@ -36,6 +36,35 @@ export function getPathPrefix(locale?: string): string {
   return '/' + segments.slice(0, localeIndex).join('/');
 }
 
+/**
+ * Mount-prefix-aware wrapper around `fetch()`.
+ *
+ * When Bulwark is served behind a reverse proxy at a sub-path (e.g. `/bulwark`),
+ * `fetch('/api/foo')` would target the browser origin at `/api/foo`, which the
+ * proxy doesn't route. `apiFetch` detects the mount prefix from
+ * `window.location.pathname` via `getPathPrefix()` at call time, so the same
+ * built bundle works at any mount point without rebuilding.
+ *
+ * Only rewrites absolute paths that start with a single `/`. Protocol-relative
+ * URLs (`//cdn.example.com/foo`) and absolute URLs (`https://...`) pass
+ * through unchanged.
+ *
+ * Server code (route handlers, layout files running at SSR) should keep using
+ * the raw Fetch API — the mount prefix is a browser-only concept.
+ *
+ * @example
+ *   await apiFetch('/api/jmap', { method: 'POST', body })
+ *   // Browser at /webmail/en/inbox  → /webmail/api/jmap
+ *   // Browser at /en/inbox          → /api/jmap
+ */
+export function apiFetch(input: string, init?: RequestInit): Promise<Response> {
+  if (input.startsWith('/') && !input.startsWith('//')) {
+    return fetch(getPathPrefix() + input, init);
+  }
+  return fetch(input, init);
+}
+
+
 /**
  * Extracts the locale from the current URL, skipping any mount prefix.
  * Falls back to 'en' when no known locale segment is found.