ci: add test coverage reporting with v8 provider

- Add vitest.config.ts with v8 coverage provider
- Add npm run test:coverage script
- Add coverage step to CI (Node 22, reports in job summary)
- Coverage tracks src/lib/ utilities (currently ~21% lines)
- Install @vitest/coverage-v8 dev dependency

package.json

@@ -32,6 +32,7 @@
     "lint": "eslint .",
     "test": "vitest run",
     "test:watch": "vitest",
+    "test:coverage": "vitest run --coverage",
     "preview": "vite preview",
     "lint:fix": "eslint . --fix"
   },
@@ -59,6 +60,7 @@
     "@types/react": "^19.2.7",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^5.1.1",
+    "@vitest/coverage-v8": "^4.0.18",
     "eslint": "^9.39.1",
     "eslint-plugin-react-hooks": "^7.0.1",
     "eslint-plugin-react-refresh": "^0.4.24",