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/favicon.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <rect width="32" height="32" rx="4" fill="#0a0a14"/>
+  <circle cx="10" cy="16" r="3" fill="#00e5ff" opacity="0.9"/>
+  <circle cx="22" cy="10" r="3" fill="#ff6644" opacity="0.9"/>
+  <circle cx="22" cy="22" r="3" fill="#44ff88" opacity="0.9"/>
+  <line x1="13" y1="16" x2="19" y2="10" stroke="#00e5ff" stroke-width="1.5" opacity="0.6"/>
+  <line x1="13" y1="16" x2="19" y2="22" stroke="#00e5ff" stroke-width="1.5" opacity="0.6"/>
+</svg>