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>

package.json

@@ -1,21 +1,10 @@
 {
-  "name": "reaktor-montlab",
-  "version": "1.0.0",
+  "name": "reaktor",
   "private": true,
-  "type": "module",
+  "workspaces": ["packages/*"],
   "scripts": {
-    "dev": "vite",
-    "build": "vite build",
-    "preview": "vite preview",
-    "start": "node server.js"
-  },
-  "dependencies": {
-    "react": "^18.2.0",
-    "react-dom": "^18.2.0",
-    "tone": "^14.8.49"
-  },
-  "devDependencies": {
-    "@vitejs/plugin-react": "^4.2.0",
-    "vite": "^5.4.0"
+    "dev": "npm run dev -w packages/client",
+    "build": "npm run build -w packages/client",
+    "start": "node packages/server/server.js"
   }
 }