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/cloud/CloudFolderTreeItem.vue

@@ -102,7 +102,7 @@ async function loadChildren() {
   loadError.value = false
   try {
     const data = await api.getCloudFolders(props.provider, props.folder.id)
-    children.value = data.items ?? []
+    children.value = (data.items ?? []).filter(i => i.is_dir)
     childrenLoaded.value = true
   } catch {
     loadError.value = true