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/package.json

@@ -14,7 +14,8 @@
     "pinia": "^3.0.4",
     "pinia-plugin-persistedstate": "^4.7.1",
     "vue": "^3.5.25",
-    "vue-router": "^5.0.2"
+    "vue-router": "^5.0.2",
+    "vuefinder": "^4.1.1"
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.1.18",