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/public/manifest.json

@@ -0,0 +1,15 @@
+{
+  "name": "Reaktor — MontLab Modular Synth",
+  "short_name": "Reaktor",
+  "description": "Modular synthesizer & SynthQuest puzzle game",
+  "start_url": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#08080f",
+  "theme_color": "#00e5ff",
+  "icons": [
+    { "src": "/favicon.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any" },
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ]
+}