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/stores/cloudConnections.js

@@ -0,0 +1,39 @@
+import { defineStore } from 'pinia'
+import { ref } from 'vue'
+import * as api from '../api/client.js'
+
+export const useCloudConnectionsStore = defineStore('cloudConnections', () => {
+  const connections = ref([])
+  const loading = ref(false)
+  const error = ref(null)
+
+  async function fetchConnections() {
+    loading.value = true
+    error.value = null
+    try {
+      const data = await api.listCloudConnections()
+      connections.value = data.items ?? []
+    } catch (e) {
+      error.value = e.message || 'Failed to load cloud connections'
+    } finally {
+      loading.value = false
+    }
+  }
+
+  async function disconnect(id) {
+    try {
+      await api.disconnectCloud(id)
+      connections.value = connections.value.filter(c => c.id !== id)
+    } catch (e) {
+      throw e
+    }
+  }
+
+  async function disconnectAll() {
+    const ids = connections.value.map(c => c.id)
+    for (const id of ids) await disconnect(id)
+    connections.value = []
+  }
+
+  return { connections, loading, error, fetchConnections, disconnect, disconnectAll }
+})