test(frontend): add Vitest unit-test harness with first brand helper

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -6,7 +6,9 @@
   "scripts": {
     "dev": "vite",
     "build": "vite build",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest",
+    "test:run": "vitest run"
   },
   "dependencies": {
     "@amplitude/unified": "^1.1.9",
@@ -27,9 +29,13 @@
     "react-router-dom": "^7.2.0"
   },
   "devDependencies": {
+    "@testing-library/jest-dom": "^6.6.0",
+    "@testing-library/react": "^16.1.0",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^6.0.1",
-    "vite": "^8.0.12"
+    "jsdom": "^25.0.0",
+    "vite": "^8.0.12",
+    "vitest": "^3.0.0"
   }
 }