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/DocumentView.vue

@@ -148,6 +148,7 @@
 <script setup>
 import { ref, computed, onMounted } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
+import { formatDate, formatSize } from '../utils/formatters.js'
 import TopicBadge from '../components/topics/TopicBadge.vue'
 import DocumentPreviewModal from '../components/documents/DocumentPreviewModal.vue'
 import { useDocumentsStore } from '../stores/documents.js'
@@ -292,15 +293,4 @@ function cancelCloudDeleteWarning() {
   showCloudDeleteWarning.value = false
 }
 
-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>