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/components/layout/AppSidebar.vue

@@ -128,17 +128,18 @@
             </svg>
           </button>
 
-          <!-- "Cloud Storage" navigates to /settings -->
-          <a
-            href="/settings"
+          <!-- "Cloud Storage" navigates to the cloud overview -->
+          <router-link
+            to="/cloud"
             class="nav-link flex-1 min-w-0"
+            :class="{ 'nav-link-active': $route.path.startsWith('/cloud') }"
           >
             <svg class="w-4 h-4 mr-2 shrink-0 text-sky-500" fill="none" stroke="currentColor" viewBox="0 0 24 24">
               <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2"
                 d="M3 15a4 4 0 004 4h9a5 5 0 10-.1-9.999 5.002 5.002 0 10-9.78 2.096A4.001 4.001 0 003 15z" />
             </svg>
             Cloud Storage
-          </a>
+          </router-link>
         </div>
 
         <!-- Collapsible content -->