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/vite.config.ts

@@ -0,0 +1,17 @@
+/// <reference types="vitest" />
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+
+export default defineConfig({
+  plugins: [react()],
+  server: {
+    proxy: {
+      "/api": "http://localhost:8080",
+    },
+  },
+  test: {
+    globals: true,
+    environment: "jsdom",
+    setupFiles: ["./tests/setup.ts"],
+  },
+});