feat(05-07): cloud connections Pinia store + API client functions

- Create useCloudConnectionsStore with connections/loading/error refs
- fetchConnections, disconnect(id), disconnectAll() actions
- Append listCloudConnections, disconnectCloud, connectWebDav, updateDefaultStorage to api/client.js
- Add vitest test script to package.json
- 4 unit tests passing (W4 — CLAUDE.md)

frontend/src/api/client.js

@@ -364,3 +364,29 @@ export function adminListAuditLog({ start, end, user_id, event_type, page = 1, p
 export function getDocumentContentUrl(docId) {
   return `/api/documents/${docId}/content`
 }
+
+// ── Cloud Storage ─────────────────────────────────────────────────────────────
+
+export function listCloudConnections() {
+  return request('/api/cloud/connections')
+}
+
+export function disconnectCloud(id) {
+  return request(`/api/cloud/connections/${id}`, { method: 'DELETE' })
+}
+
+export function connectWebDav(provider, serverUrl, username, password) {
+  return request('/api/cloud/connections/webdav', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ provider, server_url: serverUrl, username, password }),
+  })
+}
+
+export function updateDefaultStorage(backend) {
+  return request('/api/users/me/default-storage', {
+    method: 'PATCH',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ backend }),
+  })
+}