Add a thumbnail filmstrip to the photo lightbox.

- Render a horizontally scrollable strip of thumbnails below the image
  that highlights and auto-centres the active photo and lets you jump
  straight to any shot.
- Pass thumbUrl through from the public and admin galleries, falling
  back to previewUrl when no thumbnail exists.
- Restructure the overlay into a flex column (image area + strip) and
  require a clearly horizontal gesture before treating a touch as a
  prev/next swipe, so vertical scrolls no longer trigger navigation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/app/admin/photos/[id]/page.tsx

@@ -266,6 +266,7 @@ export default function AdminGalleryDetailPage() {
     previewUrl: p.urls.preview || p.urls.original,
     downloadUrl: p.urls.original,
     filename: p.originalFilename,
+    thumbUrl: p.urls.thumb,
   }));
   const openLightboxFor = (photoId: string) => {
     const idx = readyPhotos.findIndex((p) => p.id === photoId);