Add comprehensive automated testing infrastructure

Backend:
  - pytest configuration with async support and coverage
  - Unit tests for notification service (23 tests)
  - Unit tests for smart plug manager (12 tests)
  - Unit tests for archive service (16 tests)
  - Integration tests for API endpoints
  - Fix: notifications now send immediately (digest is summary only)

  Frontend:
  - Vitest configuration with jsdom and coverage
  - MSW for API mocking
  - Component tests for Toggle, Button, Card, ConfirmModal (77 tests)
  - Test utilities with custom render wrapper

  CI/CD:
  - GitHub Actions workflow for automated testing
  - Backend lint, unit tests, integration tests
  - Frontend lint, type-check, unit tests, build

frontend/package.json

@@ -7,7 +7,11 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest",
+    "test:run": "vitest run",
+    "test:coverage": "vitest run --coverage",
+    "test:ui": "vitest --ui"
   },
   "dependencies": {
     "@dnd-kit/core": "^6.3.1",
@@ -39,19 +43,26 @@
   "devDependencies": {
     "@eslint/js": "^9.39.1",
     "@tailwindcss/postcss": "^4.1.17",
+    "@testing-library/jest-dom": "^6.6.0",
+    "@testing-library/react": "^16.0.0",
+    "@testing-library/user-event": "^14.5.0",
     "@types/node": "^24.10.1",
     "@types/react": "^19.2.5",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^5.1.1",
+    "@vitest/coverage-v8": "^2.1.0",
     "autoprefixer": "^10.4.22",
     "eslint": "^9.39.1",
     "eslint-plugin-react-hooks": "^7.0.1",
     "eslint-plugin-react-refresh": "^0.4.24",
     "globals": "^16.5.0",
+    "jsdom": "^25.0.0",
+    "msw": "^2.6.0",
     "postcss": "^8.5.6",
     "tailwindcss": "^4.1.17",
     "typescript": "~5.9.3",
     "typescript-eslint": "^8.46.4",
-    "vite": "^7.2.4"
+    "vite": "^7.2.4",
+    "vitest": "^2.1.0"
   }
 }