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>

.htaccess

@@ -0,0 +1,5 @@
+RewriteEngine On
+
+# Route /api/* requests to the PHP API handler
+RewriteCond %{REQUEST_URI} ^/api/
+RewriteRule ^api/(.*)$ api/index.php [L,QSA]