feat(05): cloud folder browser views, routing, and sidebar nav

Add CloudStorageView (/cloud) and CloudFolderView (/cloud/:provider/:folderId).
Tree items filter to directories only (is_dir) to hide files in the nav tree.
CloudProviderTreeItem root click navigates to /cloud/{provider}/root instead
of /settings. AppSidebar Cloud Storage link upgraded to router-link with
active-class highlighting. Router registers both cloud routes with requiresAuth.

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

frontend/src/router/index.js

@@ -4,6 +4,8 @@ import FileManagerView from '../views/FileManagerView.vue'
 import TopicsView from '../views/TopicsView.vue'
 import DocumentView from '../views/DocumentView.vue'
 import SettingsView from '../views/SettingsView.vue'
+import CloudFolderView from '../views/CloudFolderView.vue'
+import CloudStorageView from '../views/CloudStorageView.vue'
 
 const routes = [
   // File manager is the home — handles both root and folder views
@@ -39,6 +41,20 @@ const routes = [
   { path: '/account', component: () => import('../views/AccountView.vue') },
   { path: '/admin', component: () => import('../views/AdminView.vue') },
 
+  // Cloud storage overview and folder browser
+  {
+    path: '/cloud',
+    name: 'cloud',
+    component: CloudStorageView,
+    meta: { requiresAuth: true },
+  },
+  {
+    path: '/cloud/:provider/:folderId(.*)',
+    name: 'cloud-folder',
+    component: CloudFolderView,
+    meta: { requiresAuth: true },
+  },
+
   // Phase 4 — folder and sharing routes
   {
     path: '/folders/:folderId',