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/tests/App.test.tsx

@@ -0,0 +1,10 @@
+import { render, screen } from "@testing-library/react";
+import { describe, expect, it } from "vitest";
+import App from "../src/App";
+
+describe("App", () => {
+  it("renders a main element", () => {
+    render(<App />);
+    expect(screen.getByRole("main")).toBeDefined();
+  });
+});