fix(admin): fallback adminThumbnail to original URL when tablet size missing

src/collections/Media.ts

@@ -28,7 +28,7 @@ export const Media: CollectionConfig = {
         formatOptions: { format: 'webp', options: { quality: 82 } },
       },
     ],
-    adminThumbnail: 'tablet',
+    adminThumbnail: ({ doc }) => (doc.sizes?.tablet?.url ?? doc.url) as string,
     // Конвертуємо оригінал у WebP при завантаженні
     formatOptions: { format: 'webp', options: { quality: 85 } },
     resizeOptions: { width: 1920, withoutEnlargement: true },