Add postinstall script to auto-build CSS

- Added 'postinstall': 'npm run build' to package.json
- This ensures CSS is automatically rebuilt when dependencies are installed
- Provides better developer experience and ensures CSS is always up-to-date

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

package.json

@@ -5,6 +5,7 @@
   "type": "module",
   "scripts": {
     "build": "lessc styles/scrying-pool.less dist/styles/scrying-pool.css",
+    "postinstall": "npm run build",
     "watch": "chokidar 'styles/**/*.less' -c 'lessc styles/scrying-pool.less dist/styles/scrying-pool.css'",
     "typecheck": "tsc --noEmit",
     "lint": "eslint src/ module.js scripts/ tests/",