refactor: restructure to monorepo with npm workspaces (Phase 0)

Move frontend to packages/client/, server to packages/server/.
Root package.json uses npm workspaces to orchestrate both.

Structure:
  reaktor/
    packages/client/  (React + Vite + Tone.js frontend)
    packages/server/  (static file server, future API)
    dist/             (built output, shared)
    docker-compose.yml (app + PostgreSQL for future backend)

- npm run dev → runs Vite dev server from client workspace
- npm run build → builds client, outputs to root dist/
- npm run start → runs server.js serving dist/
- Dockerfile updated for multi-stage monorepo build
- docker-compose.yml added with PostgreSQL service (ready for Phase 1)
- All imports and paths preserved, zero functionality change

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/client/src/main.jsx

@@ -0,0 +1,37 @@
+import React, { useState } from 'react';
+import { createRoot } from 'react-dom/client';
+import App from './App';
+import GameApp from './game/GameApp.jsx';
+import './index.css';
+
+function Root() {
+  const [mode, setMode] = useState('game'); // 'game' | 'sandbox'
+
+  if (mode === 'sandbox') {
+    return <App onSwitchToGame={() => setMode('game')} />;
+  }
+
+  return <GameApp onSwitchToSandbox={() => setMode('sandbox')} />;
+}
+
+createRoot(document.getElementById('root')).render(<Root />);
+
+// Configure and unlock audio context on first user interaction
+import * as Tone from 'tone';
+Tone.getContext().lookAhead = 0.05; // 50ms — tighter than default 100ms
+const unlockAudio = () => {
+  if (Tone.context.state !== 'running') {
+    Tone.start().catch(() => {});
+  }
+  document.removeEventListener('pointerdown', unlockAudio);
+  document.removeEventListener('keydown', unlockAudio);
+};
+document.addEventListener('pointerdown', unlockAudio);
+document.addEventListener('keydown', unlockAudio);
+
+// Register service worker for PWA
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    navigator.serviceWorker.register('/sw.js').catch(() => {});
+  });
+}