feat(phase-4): complete UX redesign — FileManagerView, FolderTreeItem, test suite, and all Phase 4 fixes

Adds the unified file manager view (Windows Explorer-style), collapsible
folder tree sidebar item, full vitest test suite (55 tests, 4 files), and
commits all Phase 4 backend/frontend fixes that were staged but uncommitted.

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

frontend/src/main.js

@@ -7,4 +7,7 @@ import './style.css'
 const app = createApp(App)
 app.use(createPinia())
 app.use(router)
+// Wait for the initial navigation guard (token refresh) to complete before mounting
+// so onMounted hooks in App.vue and child components have a valid access token.
+await router.isReady()
 app.mount('#app')