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/components/media/media.css.ts

@@ -5,6 +5,7 @@ export const Image = style([
   DefaultReset,
   {
     objectFit: 'cover',
+    objectPosition: 'center top',
     width: '100%',
     height: '100%',
   },