scaffold: React 19 + Spring Boot 3.4 + PostgreSQL project

Sets up toolchain pinning, format/lint (Biome + Spotless), lefthook
pre-commit hooks, Docker Compose for Postgres, and trivial passing
tests for both stacks.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -0,0 +1,33 @@
+{
+  "name": "frontend",
+  "private": true,
+  "version": "0.0.1",
+  "type": "module",
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b && vite build",
+    "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "lint": "biome check src",
+    "format": "biome format --write src"
+  },
+  "dependencies": {
+    "react": "^19.0.0",
+    "react-dom": "^19.0.0",
+    "zod": "^3.23.8"
+  },
+  "devDependencies": {
+    "@biomejs/biome": "^1.9.4",
+    "@testing-library/jest-dom": "^6.6.3",
+    "@testing-library/react": "^16.1.0",
+    "@testing-library/user-event": "^14.5.2",
+    "@types/react": "^19.0.2",
+    "@types/react-dom": "^19.0.2",
+    "@vitejs/plugin-react": "^4.3.4",
+    "jsdom": "^25.0.1",
+    "typescript": "~5.7.2",
+    "vite": "^6.0.5",
+    "vitest": "^2.1.8"
+  }
+}