Initial scaffold: pnpm workspace monorepo with backend + frontend

- Backend: Express + WebSocket proxy (tsx watch, strict TS, ESM)
- Frontend: React 19 + Vite + Monaco + Tailwind v4 + Tauri v2
- pnpm workspace at root for single-install workflow
- AGENTS.md with architectural constraints and commands
- Removed Tauri greet demo, added minimal App shell
- No external database, Git-as-Database philosophy

typst-leaf-frontend/src/App.tsx

@@ -0,0 +1,5 @@
+function App() {
+  return <div className="h-screen w-screen bg-zinc-900 text-zinc-100" />;
+}
+
+export default App;