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/package.json

@@ -8,15 +8,18 @@
     "preview": "vite preview"
   },
   "dependencies": {
+    "pinia": "^2.1.0",
     "vue": "^3.4.0",
-    "vue-router": "^4.3.0",
-    "pinia": "^2.1.0"
+    "vue-router": "^4.3.0"
   },
   "devDependencies": {
     "@vitejs/plugin-vue": "^5.0.0",
-    "vite": "^5.2.0",
-    "tailwindcss": "^3.4.0",
+    "@vue/test-utils": "^2.4.10",
+    "autoprefixer": "^10.4.0",
+    "happy-dom": "^20.9.0",
     "postcss": "^8.4.0",
-    "autoprefixer": "^10.4.0"
+    "tailwindcss": "^3.4.0",
+    "vite": "^5.2.0",
+    "vitest": "^4.1.7"
   }
 }