fix(studio): the strip asks for the frame in view, and lands where it is put

The rail glided to the frame in hand. A glide across a roll of six hundred
fires a scroll event a frame, and every one of them asked the thumbnail
cache for another window: the tiles of the frames being looked at queued
behind two screens of margin, so the strip sat blank exactly where it was
being read while it drew frames nobody had reached. The rail now lands
where it is put, one window is asked for, and the frames in view are the
first asked for a tile.

The strip also keeps its height: `overflow-x: auto` makes the other axis a
scroller too, so a strip squeezed by the column hid its own frames rather
than spilling them. The photo above gives way instead.

docker/frontend/src/Library.tsx

@@ -4237,7 +4237,7 @@ function dropThumbUrl(id: string): void {
   globalThumbUrls.delete(id);
 }
 
-function getSyncThumbUrl(photo: LibraryPhoto): string | null {
+export function getSyncThumbUrl(photo: LibraryPhoto): string | null {
   const held = globalThumbUrls.get(photo.id);
   if (held) return held.url;
   const blob = photo.thumb ?? getCachedThumb(photo.id);