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 <[email protected]>

frontend/src/pages/admin/CreateEventPageEnhanced.tsx

@@ -15,7 +15,7 @@ import { enUS, de } from 'date-fns/locale';
 import { toast } from 'react-toastify';
 
 import { Button, Input, Card } from '../../components/common';
-import { ThemeCustomizerEnhanced } from '../../components/admin/ThemeCustomizerEnhanced';
+import { ThemeCustomizerEnhanced, GalleryPreview } from '../../components/admin';
 import { useMutation, useQuery } from '@tanstack/react-query';
 import { eventsService } from '../../services/events.service';
 import { categoriesService } from '../../services/categories.service';
@@ -375,14 +375,28 @@ export const CreateEventPageEnhanced: React.FC = () => {
 
             {/* Theme Customizer */}
             {showThemeCustomizer && (
-              <ThemeCustomizerEnhanced
-                value={formData.theme_config}
-                onChange={handleThemeChange}
-                presetName={formData.theme_preset}
-                onPresetChange={handlePresetChange}
-                isPreviewMode={false}
-                showGalleryLayouts={true}
-              />
+              <div className="space-y-6">
+                <div className="grid grid-cols-1 lg:grid-cols-2 gap-6">
+                  {/* Theme Customizer */}
+                  <ThemeCustomizerEnhanced
+                    value={formData.theme_config}
+                    onChange={handleThemeChange}
+                    presetName={formData.theme_preset}
+                    onPresetChange={handlePresetChange}
+                    isPreviewMode={false}
+                    showGalleryLayouts={true}
+                    hideActions={true}
+                  />
+                  
+                  {/* Gallery Preview */}
+                  <div className="lg:sticky lg:top-4 lg:h-fit">
+                    <GalleryPreview 
+                      theme={formData.theme_config} 
+                      className="shadow-lg" 
+                    />
+                  </div>
+                </div>
+              </div>
             )}
           </div>
         </Card>