feat: replace polling with Tauri events and add unit tests

- Backend now emits playback:state events every 250ms when state changes
- PlayerStore uses listen() instead of setInterval for real-time updates
- Added Vitest testing framework with mocked Tauri APIs
- Added 53 unit tests for playerStore and queueStore

package.json

@@ -9,6 +9,8 @@
     "preview": "vite preview",
     "check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
     "check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch",
+    "test": "vitest",
+    "test:run": "vitest run",
     "tauri": "tauri"
   },
   "license": "MIT",
@@ -25,9 +27,12 @@
     "@sveltejs/kit": "^2.9.0",
     "@sveltejs/vite-plugin-svelte": "^5.0.0",
     "@tauri-apps/cli": "^2",
+    "@testing-library/svelte": "^5.3.1",
+    "jsdom": "^27.4.0",
     "svelte": "^5.0.0",
     "svelte-check": "^4.0.0",
     "typescript": "~5.6.2",
-    "vite": "^6.0.3"
+    "vite": "^6.0.3",
+    "vitest": "^4.0.16"
   }
 }