build: add Nix flake and NixOS module for reproducible builds

- flake.nix: devShell, frontend package, backend package, combined default
- spatial-office.nix: standalone NixOS module for importing into n97 config
- packages/frontend: React + @react-three/fiber + Zustand (Vite build)
- packages/backend: Node.js + ws + Zod (WebSocket server, port 18790)
- pnpm-workspace.yaml: monorepo workspace config

nix build .#frontend  — bundled static site
nix build .#backend   — runnable WebSocket server
nix build             — full spatial-office application
nix develop           — dev shell with Node.js 22 + pnpm

packages/backend/package.json

@@ -0,0 +1,21 @@
+{
+  "name": "spatial-office-backend",
+  "private": true,
+  "version": "0.1.0",
+  "type": "module",
+  "scripts": {
+    "dev": "tsx watch src/index.ts",
+    "build": "mkdir -p dist && cp src/index.ts dist/index.js",
+    "start": "node dist/index.js"
+  },
+  "dependencies": {
+    "ws": "^8.16.0",
+    "zod": "^3.22.0"
+  },
+  "devDependencies": {
+    "@types/node": "^22.0.0",
+    "@types/ws": "^8.5.0",
+    "tsx": "^4.7.0",
+    "typescript": "^5.4.0"
+  }
+}