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/components/documents/DocumentCard.vue

@@ -97,6 +97,7 @@ import { useFoldersStore } from '../../stores/folders.js'
 import { moveDocument } from '../../api/client.js'
 import TopicBadge from '../topics/TopicBadge.vue'
 import ShareModal from '../sharing/ShareModal.vue'
+import { formatDate, formatSize } from '../../utils/formatters.js'
 
 const props = defineProps({
   doc: Object,
@@ -137,15 +138,4 @@ function topicColor(name) {
   return topicsStore.topics.find(t => t.name === name)?.color ?? '#6366f1'
 }
 
-function formatDate(iso) {
-  if (!iso) return ''
-  return new Date(iso).toLocaleDateString(undefined, { month: 'short', day: 'numeric', year: 'numeric' })
-}
-
-function formatSize(bytes) {
-  if (!bytes) return ''
-  if (bytes < 1024) return bytes + ' B'
-  if (bytes < 1024 * 1024) return (bytes / 1024).toFixed(1) + ' KB'
-  return (bytes / (1024 * 1024)).toFixed(1) + ' MB'
-}
 </script>