Phase 4: Theme Engine - Standard stack: Pebble 4.1.1 + pebble-scala for Twig-compatible templates - Asset bundling: Vite with manifest-based backend integration - Hot reload: Mill --watch + classloader-based reload - Vue hydration: createSSRApp with server-rendered HTML - Caching: ZCaffeine for static page caching - Architecture patterns documented for layouts, partials, rendering modes