fix: resolve external media dimensions, gallery theme race condition, and add email color customization

- Fix dimension repair for external media by using photoResolver instead of hardcoded paths
- Extract photo dimensions via Sharp during external media import
- Remove duplicate theme useEffect from GalleryView to prevent flash/revert race condition
- Pass event welcome_message to Story layout footer for per-event customization
- Add email_primary_color/email_secondary_color settings with admin UI color pickers
- Add i18n keys for email branding in all 4 locales (en, de, ru, pt)

frontend/src/services/settings.service.ts

@@ -242,6 +242,8 @@ export const settingsService = {
       endpoint = '/admin/settings/branding';
     } else if (firstKey?.startsWith('seo_')) {
       endpoint = '/admin/settings/seo';
+    } else if (firstKey?.startsWith('email_')) {
+      endpoint = '/admin/settings/general';
     }
     
     await api.put(endpoint, settings);