Add Vitest coverage for frontend tests

- Install @vitest/coverage-v8
- Configure coverage in vitest.config.ts
- Add npm script: test:coverage
- Add coverage/ to gitignore

frontend/package.json

@@ -8,6 +8,7 @@
     "build": "next build",
     "start": "next start",
     "test": "vitest run",
+    "test:coverage": "vitest run --coverage",
     "test:e2e": "playwright test",
     "generate-api-types": "openapi-typescript http://localhost:8000/openapi.json -o app/generated/api.ts",
     "lint": "eslint .",
@@ -28,6 +29,7 @@
     "@types/node": "25.0.3",
     "@types/react": "19.2.7",
     "@vitejs/plugin-react": "^4.3.4",
+    "@vitest/coverage-v8": "^2.1.9",
     "eslint": "^9.39.2",
     "eslint-plugin-react-hooks": "^7.0.1",
     "jsdom": "^26.0.0",