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/[locale]/login/page.tsx

@@ -10,7 +10,7 @@ import { useAuthStore } from "@/stores/auth-store";
 import { useThemeStore } from "@/stores/theme-store";
 import { useShallow } from "zustand/react/shallow";
 import { useConfig } from "@/hooks/use-config";
-import { getPathPrefix } from "@/lib/browser-navigation";
+import { apiFetch, getPathPrefix } from "@/lib/browser-navigation";
 import { cn } from "@/lib/utils";
 import { AlertCircle, Loader2, X, Info, Eye, EyeOff, LogIn, Sun, Moon, Monitor, Check, Shield, Play, Copy } from "lucide-react";
 import { discoverOAuth, type OAuthMetadata } from "@/lib/oauth/discovery";
@@ -234,7 +234,7 @@ export default function LoginPage() {
     try {
       const prefix = getPathPrefix(params.locale as string);
       const redirectUri = `${window.location.origin}${prefix}/${params.locale}/auth/callback`;
-      const res = await fetch('/api/auth/sso/start', {
+      const res = await apiFetch('/api/auth/sso/start', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
         credentials: 'include',