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/utils/formatters.js

@@ -0,0 +1,52 @@
+/**
+ * Shared formatting utilities — import from here, never redefine inline.
+ */
+
+export function formatDate(iso) {
+  if (!iso) return '—'
+  return new Date(iso).toLocaleDateString(undefined, {
+    month: 'short',
+    day: 'numeric',
+    year: 'numeric',
+  })
+}
+
+export function formatSize(bytes) {
+  if (!bytes) return '—'
+  if (bytes < 1024) return bytes + ' B'
+  if (bytes < 1048576) return (bytes / 1024).toFixed(1) + ' KB'
+  return (bytes / 1048576).toFixed(1) + ' MB'
+}
+
+/** Maps a cloud provider slug to a Tailwind text-color class. */
+export function providerColor(provider) {
+  const map = {
+    google_drive: 'text-blue-500',
+    onedrive: 'text-sky-500',
+    nextcloud: 'text-orange-500',
+    webdav: 'text-gray-500',
+  }
+  return map[provider] ?? 'text-gray-400'
+}
+
+/** Maps a cloud provider slug to a Tailwind background-color class. */
+export function providerBg(provider) {
+  const map = {
+    google_drive: 'bg-blue-50',
+    onedrive: 'bg-sky-50',
+    nextcloud: 'bg-orange-50',
+    webdav: 'bg-gray-50',
+  }
+  return map[provider] ?? 'bg-gray-50'
+}
+
+/** Human-readable label for a cloud provider slug. */
+export function providerLabel(provider) {
+  const map = {
+    google_drive: 'Google Drive',
+    onedrive: 'OneDrive',
+    nextcloud: 'Nextcloud',
+    webdav: 'WebDAV',
+  }
+  return map[provider] ?? provider
+}