perf(media): center-top focal point on cover-fit thumbnails (P5-6)

Adds objectPosition:'center top' to all cover-fit thumbnail surfaces so
portrait images show faces/subjects instead of the center-slice when
the 600px AttachmentBox height cap forces cropping.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/room/MediaGallery.tsx

@@ -422,7 +422,7 @@ function GalleryTile({
         <img
           src={media.url}
           alt={body}
-          style={{ width: '100%', height: '100%', objectFit: 'cover', display: 'block' }}
+          style={{ width: '100%', height: '100%', objectFit: 'cover', objectPosition: 'center top', display: 'block' }}
         />
       )}