Initial commit: Site Builder with PHP API backend

Visual drag-and-drop website builder using GrapesJS with:
- Multi-page editor with live preview
- File-based asset storage via PHP API (no localStorage base64)
- Template library, Docker support, and Playwright test suite

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,9 @@
+services:
+  site-builder:
+    image: nginx:alpine
+    container_name: site-builder
+    ports:
+      - "8081:80"
+    volumes:
+      - .:/usr/share/nginx/html:ro
+    restart: unless-stopped