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/router/index.ts

@@ -105,6 +105,11 @@ const panelRoutes: RouteRecordRaw[] = [
         name: 'plugins',
         component: () => import('@/views/admin/PluginsView.vue'),
       },
+      {
+        path: 'files',
+        name: 'files',
+        component: () => import('@/views/admin/FileManagerView.vue'),
+      },
       {
         path: 'wipes',
         name: 'wipes',