Stabilize knowledge uploads in the UI

Keep folder selection stable across refreshes, surface upload failures
more clearly, and add focused composable tests for the knowledge page.
This keeps newly uploaded files visible and makes MinerU dependency
errors easier to understand from the frontend.

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

frontend/package.json

@@ -6,7 +6,8 @@
   "scripts": {
     "dev": "vite",
     "build": "vue-tsc -b && vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run"
   },
   "dependencies": {
     "@vueuse/core": "^14.2.1",
@@ -22,9 +23,12 @@
   "devDependencies": {
     "@types/node": "^25.5.0",
     "@vitejs/plugin-vue": "^6.0.5",
+    "@vue/test-utils": "^2.4.6",
     "@vue/tsconfig": "^0.9.0",
+    "jsdom": "^29.0.1",
     "typescript": "^5.9.3",
     "vite": "^8.0.1",
+    "vitest": "^4.1.0",
     "vue-tsc": "^2.2.12"
   }
 }