feat: Add file manager view using VueFinder

Installs VueFinder and wires it to the backend /api/files endpoint with
JWT Bearer auth. Adds /files route, File Manager nav item (files.view
permission-gated, FolderOpen icon), and imports VueFinder CSS globally.
Driver token is computed reactively so it tracks token refreshes automatically.
Uses midnight theme to match the dark admin panel aesthetic.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/main.ts

@@ -5,6 +5,7 @@ import piniaPluginPersistedstate from 'pinia-plugin-persistedstate'
 import App from './App.vue'
 import router from './router'
 import './style.css'
+import 'vuefinder/dist/vuefinder.css'
 
 const app = createApp(App)