refactor(frontend): extract shared modules, thin views, delete dead code

Shared utilities:
- Add src/utils/formatters.js — formatDate, formatSize, providerColor,
  providerBg, providerLabel; all components import from here, no inline duplicates
- Add src/components/ui/TreeItem.vue — generic expand/collapse tree node;
  FolderTreeItem, CloudFolderTreeItem, CloudProviderTreeItem now wrap it
- Add src/components/storage/StorageBrowser.vue — unified file browser grid
  used by both FileManagerView and CloudFolderView

View refactor (thin data-providers):
- FileManagerView.vue: stripped to props + event wiring; all layout moved to StorageBrowser
- CloudFolderView.vue: same treatment — feeds props into StorageBrowser
- All tree sidebar components delegate expand/collapse to TreeItem.vue

Dead code removed:
- Delete HomeView.vue — no active route, replaced by FileManagerView
- Delete FolderView.vue — no active route, logic merged into FileManagerView

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

frontend/src/views/CloudStorageView.vue

@@ -62,6 +62,7 @@
 import { computed } from 'vue'
 import { useRouter } from 'vue-router'
 import { useCloudConnectionsStore } from '../stores/cloudConnections.js'
+import { providerColor, providerBg } from '../utils/formatters.js'
 
 const router = useRouter()
 const cloudStore = useCloudConnectionsStore()
@@ -73,21 +74,4 @@ function openProvider(conn) {
   router.push(`/cloud/${conn.provider}/root`)
 }
 
-function providerColor(provider) {
-  return {
-    google_drive: 'text-blue-500',
-    onedrive: 'text-sky-500',
-    nextcloud: 'text-orange-500',
-    webdav: 'text-gray-500',
-  }[provider] ?? 'text-gray-400'
-}
-
-function providerBg(provider) {
-  return {
-    google_drive: 'bg-blue-50',
-    onedrive: 'bg-sky-50',
-    nextcloud: 'bg-orange-50',
-    webdav: 'bg-gray-100',
-  }[provider] ?? 'bg-gray-50'
-}
 </script>