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>

Dockerfile

@@ -1,13 +1,17 @@
 FROM node:20-alpine AS build
 WORKDIR /app
 COPY package.json package-lock.json* ./
+COPY packages/client/package.json packages/client/
+COPY packages/server/package.json packages/server/
 RUN npm install
-COPY . .
+COPY packages/client packages/client
 RUN npm run build
 
 FROM node:20-alpine
 WORKDIR /app
 COPY --from=build /app/dist ./dist
-COPY server.js .
+COPY packages/server/server.js ./packages/server/server.js
+COPY packages/server/package.json ./packages/server/package.json
+COPY package.json ./
 EXPOSE 80
-CMD ["node", "server.js"]
+CMD ["node", "packages/server/server.js"]