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/i18n/locales/en.json

@@ -396,6 +396,10 @@
       "language": "Language",
       "defaultLanguage": "Default Language",
       "defaultLanguageHelp": "Language shown to guests before login",
+      "defaultWelcomeMessage": "Default Welcome Message",
+      "welcomeMessage": "Welcome Message",
+      "welcomeMessagePlaceholder": "Enter a default welcome message that will be included in gallery creation emails",
+      "welcomeMessageHelp": "This message will be included in all gallery creation emails unless overridden when creating an event",
       "saveSettings": "Save General Settings",
       "saveGeneralSettings": "Save General Settings",
       "dateTimeFormat": "Date & Time Format",
@@ -634,7 +638,9 @@
     "applyTheme": "Apply Theme",
     "customTheme": "Custom Theme",
     "customizeTheme": "Customize Theme",
-    "saveTheme": "Save Theme"
+    "saveTheme": "Save Theme",
+    "previewLayout": "Preview Layout",
+    "livePreview": "Live Preview"
   },
   "admin": {
     "title": "Admin Panel",