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/sw.js

@@ -0,0 +1,33 @@
+const CACHE_NAME = 'reaktor-v1';
+
+self.addEventListener('install', (e) => {
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', (e) => {
+  e.waitUntil(
+    caches.keys().then(keys =>
+      Promise.all(keys.filter(k => k !== CACHE_NAME).map(k => caches.delete(k)))
+    )
+  );
+  self.clients.claim();
+});
+
+self.addEventListener('fetch', (e) => {
+  // Only cache GET requests, skip API calls
+  if (e.request.method !== 'GET') return;
+
+  e.respondWith(
+    caches.match(e.request).then(cached => {
+      const fetching = fetch(e.request).then(response => {
+        if (response.ok) {
+          const clone = response.clone();
+          caches.open(CACHE_NAME).then(cache => cache.put(e.request, clone));
+        }
+        return response;
+      }).catch(() => cached);
+
+      return cached || fetching;
+    })
+  );
+});