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>

.user.ini

@@ -0,0 +1,5 @@
+; PHP upload/memory limits for Site Builder
+upload_max_filesize = 500M
+post_max_size = 512M
+memory_limit = 768M
+max_execution_time = 300