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>

docker-compose.yml

@@ -0,0 +1,24 @@
+services:
+  app:
+    build: .
+    ports:
+      - "80:80"
+    depends_on:
+      db:
+        condition: service_healthy
+
+  db:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: reaktor
+      POSTGRES_PASSWORD: reaktor_dev
+      POSTGRES_DB: reaktor
+    volumes:
+      - pgdata:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U reaktor"]
+      interval: 5s
+      retries: 5
+
+volumes:
+  pgdata: