feat(queue): implement concurrent deduplication and safe backend detachment

- Add backendRegisteredIds and backendDispatchPromises for single dispatch enforcement

- Add detach_download_for_reconfigure to safely modify properties of active downloads

- Add applyProperties logic handling completed, failed, paused, and active queues

- Add extractValidDownloadUrls and fix multi-url paste handling

- Setup vitest and add useDownloadStore unit tests for backend registration lifecycle

package.json

@@ -31,7 +31,8 @@
     "bindings": "cd src-tauri && cargo test export_bindings --lib",
     "build": "tsc && vite build",
     "preview": "vite preview",
-    "tauri": "tauri"
+    "tauri": "tauri",
+    "test": "vitest"
   },
   "dependencies": {
     "@tailwindcss/vite": "^4.3.0",
@@ -55,6 +56,7 @@
     "postcss": "^8.5.15",
     "tailwindcss": "^4.3.0",
     "typescript": "~5.8.3",
-    "vite": "^8.0.16"
+    "vite": "^8.0.16",
+    "vitest": "^4.1.9"
   }
 }