feat(05-08): add cloud tree components and getCloudFolders API function

- Add getCloudFolders(provider, folderId) to api/client.js (GET /api/cloud/folders/{provider}/{folderId})
- Create CloudProviderTreeItem.vue: lazy-load folder tree per connection, providerIconColor computed, expand/collapse arrow, loading/error states
- Create CloudFolderTreeItem.vue: recursive folder tree node with is_dir expand arrow, lazy-load children, depth padding

frontend/src/api/client.js

@@ -390,3 +390,7 @@ export function updateDefaultStorage(backend) {
     body: JSON.stringify({ backend }),
   })
 }
+
+export function getCloudFolders(provider, folderId) {
+  return request(`/api/cloud/folders/${provider}/${folderId}`)
+}

frontend/src/components/cloud/CloudFolderTreeItem.vue

@@ -0,0 +1,128 @@
+<template>
+  <div>
+    <!-- Row -->
+    <div
+      class="flex items-center group"
+      :style="{ paddingLeft: `${depth * 12}px` }"
+    >
+      <!-- Expand/collapse arrow (only for directories) -->
+      <button
+        v-if="folder.is_dir"
+        @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 ' + folder.name : 'Expand ' + folder.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>
+      <!-- Spacer for non-directory items -->
+      <span v-else class="w-5 h-5 shrink-0"></span>
+
+      <!-- Folder/file name button -->
+      <button
+        @click="navigateTo"
+        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"
+      >
+        <!-- Folder icon for directories, document icon for files -->
+        <svg
+          v-if="folder.is_dir"
+          class="w-4 h-4 shrink-0 text-gray-400"
+          fill="none"
+          stroke="currentColor"
+          viewBox="0 0 24 24"
+        >
+          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2"
+            d="M3 7a2 2 0 012-2h4l2 2h8a2 2 0 012 2v9a2 2 0 01-2 2H5a2 2 0 01-2-2V7z" />
+        </svg>
+        <svg
+          v-else
+          class="w-4 h-4 shrink-0 text-gray-400"
+          fill="none"
+          stroke="currentColor"
+          viewBox="0 0 24 24"
+        >
+          <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2"
+            d="M7 21h10a2 2 0 002-2V9.414a1 1 0 00-.293-.707l-5.414-5.414A1 1 0 0012.586 3H7a2 2 0 00-2 2v14a2 2 0 002 2z" />
+        </svg>
+        <span class="truncate">{{ folder.name }}</span>
+      </button>
+    </div>
+
+    <!-- Children: nested sub-folders (lazy loaded) -->
+    <template v-if="expanded">
+      <div v-if="loading" class="text-xs text-gray-400 py-1" :style="{ paddingLeft: `${(depth + 1) * 12 + 8}px` }">Loading…</div>
+      <div
+        v-else-if="loadError"
+        class="text-xs text-red-500 cursor-pointer py-1"
+        :style="{ paddingLeft: `${(depth + 1) * 12 + 8}px` }"
+        @click="retry"
+      >
+        Failed to load — tap to retry
+      </div>
+      <div v-else-if="children.length === 0" class="text-xs text-gray-400 py-1" :style="{ paddingLeft: `${(depth + 1) * 12 + 8}px` }">Empty</div>
+      <CloudFolderTreeItem
+        v-for="child in children"
+        :key="child.id"
+        :folder="child"
+        :provider="provider"
+        :depth="depth + 1"
+      />
+    </template>
+  </div>
+</template>
+
+<script setup>
+import { ref } from 'vue'
+import { useRouter } from 'vue-router'
+import * as api from '../../api/client.js'
+
+const props = defineProps({
+  folder: { type: Object, required: true },
+  provider: { type: String, required: true },
+  depth: { type: Number, default: 2 },
+})
+
+const router = useRouter()
+
+const expanded = ref(false)
+const children = ref([])
+const loading = ref(false)
+const loadError = ref(false)
+const childrenLoaded = ref(false)
+
+async function loadChildren() {
+  loading.value = true
+  loadError.value = false
+  try {
+    const data = await api.getCloudFolders(props.provider, props.folder.id)
+    children.value = data.items ?? []
+    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()
+}
+
+function navigateTo() {
+  router.push(`/cloud/${props.provider}/${props.folder.id}`)
+}
+</script>

frontend/src/components/cloud/CloudProviderTreeItem.vue

@@ -0,0 +1,118 @@
+<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>
+      <CloudFolderTreeItem
+        v-for="folder in children"
+        :key="folder.id"
+        :folder="folder"
+        :provider="connection.provider"
+        :depth="depth + 1"
+      />
+    </template>
+  </div>
+</template>
+
+<script setup>
+import { ref, computed } from 'vue'
+import { useRouter } from 'vue-router'
+import * as api from '../../api/client.js'
+import CloudFolderTreeItem from './CloudFolderTreeItem.vue'
+
+const props = defineProps({
+  connection: { type: Object, required: true },
+  depth: { type: Number, default: 1 },
+})
+
+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'
+})
+
+async function loadChildren() {
+  loading.value = true
+  loadError.value = false
+  try {
+    const data = await api.getCloudFolders(props.connection.provider, 'root')
+    children.value = data.items ?? []
+    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()
+}
+
+function navigateToRoot() {
+  router.push('/settings')
+}
+</script>