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/src/utils/bezier.js

@@ -0,0 +1,8 @@
+/**
+ * Generate SVG bezier path between two points (for wires)
+ */
+export function wirePath(x1, y1, x2, y2) {
+  const dx = Math.abs(x2 - x1);
+  const cp = Math.max(50, dx * 0.5);
+  return `M ${x1} ${y1} C ${x1 + cp} ${y1}, ${x2 - cp} ${y2}, ${x2} ${y2}`;
+}