feat: add testing infrastructure and improve component feedback

- Added React testing setup with JSDOM
- Added component tests for AddTransactionForm and TransactionTable
- Improved error handling and success messages in components
- Updated test configuration and dependencies
- Added CSS for error and success states

package.json

@@ -7,7 +7,8 @@
     "build": "astro build",
     "preview": "astro preview",
     "astro": "astro",
-    "test": "vitest",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "test:coverage": "vitest run --coverage",
     "format": "biome format --write .",
     "lint": "biome lint .",
@@ -27,8 +28,13 @@
   },
   "devDependencies": {
     "@biomejs/biome": "^1.9.4",
+    "@testing-library/dom": "^10.4.0",
+    "@testing-library/jest-dom": "^6.6.3",
+    "@testing-library/react": "^16.3.0",
     "@types/supertest": "^6.0.3",
-    "@vitest/coverage-v8": "^3.1.2",
+    "@vitejs/plugin-react": "^4.4.1",
+    "@vitest/coverage-v8": "^3.1.3",
+    "jsdom": "^26.1.0",
     "supertest": "^7.1.0",
     "vitest": "^3.1.2",
     "wrangler": "^4.13.1"