feat: implement gallery preview with layout selector

- Add GalleryPreview component that shows simplified gallery layouts
- Update ThemeEditorModal with split view: theme customizer on left, preview on right
- Add grid style selector above preview to switch between layouts
- Update BrandingPage to show live preview alongside theme customizer
- Add preview to CreateEventPageEnhanced when customizing themes
- Support all 6 gallery layouts: grid, masonry, carousel, timeline, hero, mosaic
- Add translation keys for preview layout and live preview

The preview accurately reflects different grid layouts and theme settings,
helping users visualize how their galleries will look before saving.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/src/components/admin/index.ts

@@ -20,4 +20,5 @@ export { ThemeCustomizerEnhanced } from './ThemeCustomizerEnhanced';
 export { ThemeDisplay } from './ThemeDisplay';
 export { ThemeEditorModal } from './ThemeEditorModal';
 export { HeroPhotoSelector } from './HeroPhotoSelector';
-export { PhotoUploadModal } from './PhotoUploadModal';
+export { PhotoUploadModal } from './PhotoUploadModal';
+export { GalleryPreview } from './GalleryPreview';