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/router/index.js

@@ -1,13 +1,13 @@
 import { createRouter, createWebHistory } from 'vue-router'
 import { useAuthStore } from '../stores/auth.js'
-import HomeView from '../views/HomeView.vue'
+import FileManagerView from '../views/FileManagerView.vue'
 import TopicsView from '../views/TopicsView.vue'
 import DocumentView from '../views/DocumentView.vue'
 import SettingsView from '../views/SettingsView.vue'
 
 const routes = [
-  // Existing routes
-  { path: '/', component: HomeView },
+  // File manager is the home — handles both root and folder views
+  { path: '/', component: FileManagerView },
   { path: '/topics', component: TopicsView },
   { path: '/topics/:name', component: TopicsView },
   { path: '/document/:id', component: DocumentView },
@@ -43,7 +43,7 @@ const routes = [
   {
     path: '/folders/:folderId',
     name: 'folder',
-    component: () => import('../views/FolderView.vue'),
+    component: FileManagerView,
     meta: { requiresAuth: true },
   },
   {