feat: add Vitest unit testing infrastructure with initial test suite.

package.json

@@ -7,7 +7,11 @@
     "dev": "vite",
     "build": "tsc -b && vite build",
     "lint": "eslint .",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest",
+    "test:ui": "vitest --ui",
+    "test:coverage": "vitest --coverage",
+    "test:run": "vitest run"
   },
   "dependencies": {
     "class-transformer": "^0.5.1",
@@ -24,20 +28,27 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.29.0",
+    "@semantic-release/changelog": "^6.0.3",
+    "@semantic-release/git": "^10.0.1",
+    "@testing-library/jest-dom": "^6.8.0",
+    "@testing-library/react": "^16.3.0",
+    "@testing-library/user-event": "^14.6.1",
     "@types/node": "^22.9.3",
     "@types/react": "^19.1.8",
     "@types/react-dom": "^19.1.6",
     "@types/react-syntax-highlighter": "^15.5.13",
     "@vitejs/plugin-react": "^4.5.2",
-    "@semantic-release/changelog": "^6.0.3",
-    "@semantic-release/git": "^10.0.1",
+    "@vitest/coverage-v8": "^3.2.4",
+    "@vitest/ui": "^3.2.4",
     "eslint": "^9.29.0",
     "eslint-plugin-react-hooks": "^5.2.0",
     "eslint-plugin-react-refresh": "^0.4.20",
     "globals": "^16.2.0",
+    "jsdom": "^26.1.0",
     "semantic-release": "^21.1.2",
     "typescript": "~5.8.3",
     "typescript-eslint": "^8.34.1",
-    "vite": "^7.0.0"
+    "vite": "^7.0.0",
+    "vitest": "^3.2.4"
   }
 }