chore(phase-4): UAT complete — Phase 4 marked done, sidebar collapse, duplicate-folder fix

UAT: 14/15 passed. Bug fixed: folders/rootFolders array alias in fetchFolders caused
duplicate folder row on creation (rootFolders = [...list] breaks the shared reference).
Sidebar: Folders section now has a collapse/expand chevron, collapsed by default.
State: Phase 4 complete, Phase 5 (Cloud Storage Backends) is next.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/stores/folders.js

@@ -18,7 +18,7 @@ export const useFoldersStore = defineStore('folders', () => {
       const data = await api.listFolders(parentId)
       const list = data.items ?? data
       folders.value = list
-      if (parentId === null) rootFolders.value = list
+      if (parentId === null) rootFolders.value = [...list]
     } catch (e) {
       error.value = e.message || 'Failed to load folders'
     } finally {