fix: display welcome message in gallery and fix guest thumbnail URLs (#306, #307)

- Render welcome_message in gallery view for all non-fullpage layouts
  (grid, masonry, carousel, timeline, mosaic) as a centered banner
- Add /api prefix to thumbnail/photo URLs in adminGuests.js and
  adminPhotos.js so they route correctly through Nginx proxy

backend/src/routes/adminGuests.js

@@ -138,8 +138,8 @@ router.get(
             id: p.id,
             filename: p.filename,
             original_filename: p.original_filename,
-            url: `/admin/photos/${eventId}/photo/${p.id}`,
-            thumbnail_url: `/admin/photos/${eventId}/thumbnail/${p.id}`,
+            url: `/api/admin/photos/${eventId}/photo/${p.id}`,
+            thumbnail_url: `/api/admin/photos/${eventId}/thumbnail/${p.id}`,
             picker_count: parseInt(p.picker_count, 10),
           })),
       });
@@ -418,8 +418,8 @@ router.get(
         filename: row.filename,
         original_filename: row.original_filename,
         type: row.type,
-        url: `/admin/photos/${eventId}/photo/${row.photo_id}`,
-        thumbnail_url: `/admin/photos/${eventId}/thumbnail/${row.photo_id}`,
+        url: `/api/admin/photos/${eventId}/photo/${row.photo_id}`,
+        thumbnail_url: `/api/admin/photos/${eventId}/thumbnail/${row.photo_id}`,
       });
 
       const selections = {

backend/src/routes/adminPhotos.js

@@ -922,9 +922,9 @@ router.get('/:eventId/photos', adminAuth, requirePermission('photos.view'), requ
         filename: photo.filename,
         original_filename: photo.original_filename || null,
         // Use the correct admin photos router base for serving images
-        url: `/admin/photos/${eventId}/photo/${photo.id}`,
+        url: `/api/admin/photos/${eventId}/photo/${photo.id}`,
         // Always expose a thumbnail URL; backend will generate on demand if missing
-        thumbnail_url: `/admin/photos/${eventId}/thumbnail/${photo.id}`,
+        thumbnail_url: `/api/admin/photos/${eventId}/thumbnail/${photo.id}`,
         type: photo.type,
         category_id: photo.category_id || photo.type,
         category_name: photo.pc_name || (photo.type === 'individual' ? 'Individual Photos' : 'Collages'),