Update postinstall script per mentor feedback (#130)

- Changed postinstall to use fs.cpSync as requested: node -e 'fs.cpSync(\"./node_modules/prismjs/themes/\", \"./src/static/css/prismjs/\", { recursive: true })'.
- Restored yarn.lock to match upstream/main to avoid unintended changes.
- Tested on Windows 11 with Node.js 22.14.0; noted quoting issue in PowerShell.

Author: ahmedhesein1

package.json

@@ -42,7 +42,7 @@
     "start": "node src/index.js",
     "lint": "prettier --check \"src/**/*.js\" \"src/**/*.css\"",
     "prettier:write": "prettier --write \"src/**/*.js\" \"src/**/*.css\"",
-    "postinstall": "cp node_modules/prismjs/themes/*.css src/static/css/prismjs",
+    "postinstall": "node -e \"require('fs').cpSync('./node_modules/prismjs/themes/', './src/static/css/prismjs/', { recursive: true })\"",
     "prepare": "husky"
   }
 }