Add mobile save sheet with same-origin photo downloads.

Phones cannot land downloads in the photo library, so the gallery opens a share sheet for a cacheable preview while streaming via a dedicated download endpoint and recording preview sizes.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/components/Lightbox.tsx

@@ -179,10 +179,14 @@ export default function Lightbox({
         )}
 
         {/* eslint-disable-next-line @next/next/no-img-element */}
+        {/* Nothing is layered over the image (the navigation buttons sit at
+            the edges), and neither -webkit-touch-callout nor -webkit-user-
+            select is suppressed here, so iOS long-press → "Save to Photos"
+            still works as a backup to the save sheet. Don't add select-none. */}
         <img
           src={item.previewUrl}
           alt=""
-          className="max-w-[95vw] max-h-full object-contain select-none"
+          className="max-w-[95vw] max-h-full object-contain [-webkit-touch-callout:default]"
           onClick={(e) => e.stopPropagation()}
           draggable={false}
         />