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-lock.json

@@ -1,21 +1,14 @@
 {
-  "name": "reaktor-montlab",
+  "name": "reaktor",
   "version": "1.0.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
-      "name": "reaktor-montlab",
-      "version": "1.0.0",
-      "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"
-      }
+      "name": "reaktor",
+      "workspaces": [
+        "packages/*"
+      ]
     },
     "node_modules/@babel/code-frame": {
       "version": "7.29.0",
@@ -749,6 +742,14 @@
         "@jridgewell/sourcemap-codec": "^1.4.14"
       }
     },
+    "node_modules/@reaktor/client": {
+      "resolved": "packages/client",
+      "link": true
+    },
+    "node_modules/@reaktor/server": {
+      "resolved": "packages/server",
+      "link": true
+    },
     "node_modules/@rolldown/pluginutils": {
       "version": "1.0.0-beta.27",
       "resolved": "https://registry.npmjs.org/@rolldown/pluginutils/-/pluginutils-1.0.0-beta.27.tgz",
@@ -1722,6 +1723,23 @@
       "integrity": "sha512-a4UGQaWPH59mOXUYnAG2ewncQS4i4F43Tv3JoAM+s2VDAmS9NsK8GpDMLrCHPksFT7h3K6TOoUNn2pb7RoXx4g==",
       "dev": true,
       "license": "ISC"
+    },
+    "packages/client": {
+      "name": "@reaktor/client",
+      "version": "1.0.0",
+      "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"
+      }
+    },
+    "packages/server": {
+      "name": "@reaktor/server",
+      "version": "1.0.0"
     }
   }
 }