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/cloud/CloudProviderTreeItem.vue

@@ -1,52 +1,17 @@
 <template>
-  <div>
-    <!-- Row -->
-    <div
-      class="flex items-center group"
-      :style="{ paddingLeft: `${depth * 12}px` }"
-    >
-      <!-- Expand/collapse arrow -->
-      <button
-        @click.prevent.stop="toggleExpand"
-        class="w-5 h-5 flex items-center justify-center text-gray-400 hover:text-gray-600 shrink-0 transition-colors"
-        :aria-label="expanded ? 'Collapse ' + connection.display_name : 'Expand ' + connection.display_name"
-      >
-        <svg
-          class="w-3 h-3 transition-transform duration-150"
-          :class="{ 'rotate-90': expanded }"
-          fill="none"
-          stroke="currentColor"
-          viewBox="0 0 24 24"
-        >
-          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2.5" d="M9 5l7 7-7 7" />
-        </svg>
-      </button>
-
-      <!-- Provider name (click navigates to /settings) -->
-      <button
-        @click="navigateToRoot"
-        class="flex items-center gap-2 flex-1 min-w-0 px-2 py-1.5 rounded-lg text-sm font-medium transition-colors text-gray-600 hover:bg-gray-100 hover:text-gray-900"
-      >
-        <!-- Provider cloud icon (w-4 h-4, provider color) -->
-        <svg class="w-4 h-4 shrink-0" :class="providerIconColor" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2"
-            d="M3 15a4 4 0 004 4h9a5 5 0 10-.1-9.999 5.002 5.002 0 10-9.78 2.096A4.001 4.001 0 003 15z" />
-        </svg>
-        <span class="truncate">{{ connection.display_name }}</span>
-      </button>
-    </div>
-
-    <!-- Children: first-level cloud folders (lazy loaded) -->
-    <template v-if="expanded">
-      <div v-if="loading" class="pl-12 py-1 text-xs text-gray-400">Loading…</div>
-      <div
-        v-else-if="loadError"
-        class="pl-12 py-1 text-xs text-red-500 cursor-pointer"
-        @click="retry"
-      >
-        Failed to load — tap to retry
-      </div>
-      <div v-else-if="children.length === 0" class="pl-12 py-1 text-xs text-gray-400">Empty</div>
+  <TreeItem
+    :label="connection.display_name"
+    :load-children="loadChildren"
+    :depth="depth"
+    @select="navigateToRoot"
+  >
+    <template #icon>
+      <svg class="w-4 h-4 shrink-0" :class="providerIconColor" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2"
+          d="M3 15a4 4 0 004 4h9a5 5 0 10-.1-9.999 5.002 5.002 0 10-9.78 2.096A4.001 4.001 0 003 15z" />
+      </svg>
+    </template>
+    <template #children="{ children }">
       <CloudFolderTreeItem
         v-for="folder in children"
         :key="folder.id"
@@ -55,14 +20,16 @@
         :depth="depth + 1"
       />
     </template>
-  </div>
+  </TreeItem>
 </template>
 
 <script setup>
-import { ref, computed } from 'vue'
+import { computed } from 'vue'
 import { useRouter } from 'vue-router'
 import * as api from '../../api/client.js'
+import TreeItem from '../ui/TreeItem.vue'
 import CloudFolderTreeItem from './CloudFolderTreeItem.vue'
+import { providerColor } from '../../utils/formatters.js'
 
 const props = defineProps({
   connection: { type: Object, required: true },
@@ -71,45 +38,11 @@ const props = defineProps({
 
 const router = useRouter()
 
-const expanded = ref(false)
-const children = ref([])
-const loading = ref(false)
-const loadError = ref(false)
-const childrenLoaded = ref(false)
-
-const providerIconColor = computed(() => {
-  const map = {
-    google_drive: 'text-blue-500',
-    onedrive: 'text-sky-500',
-    nextcloud: 'text-orange-500',
-    webdav: 'text-gray-500',
-  }
-  return map[props.connection.provider] ?? 'text-gray-400'
-})
+const providerIconColor = computed(() => providerColor(props.connection.provider))
 
 async function loadChildren() {
-  loading.value = true
-  loadError.value = false
-  try {
-    const data = await api.getCloudFolders(props.connection.provider, 'root')
-    children.value = (data.items ?? []).filter(i => i.is_dir)
-    childrenLoaded.value = true
-  } catch {
-    loadError.value = true
-  } finally {
-    loading.value = false
-  }
-}
-
-async function toggleExpand() {
-  if (!expanded.value && !childrenLoaded.value) {
-    await loadChildren()
-  }
-  expanded.value = !expanded.value
-}
-
-async function retry() {
-  await loadChildren()
+  const data = await api.getCloudFolders(props.connection.provider, 'root')
+  return (data.items ?? []).filter(i => i.is_dir)
 }
 
 function navigateToRoot() {