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/RenderMessageContent.tsx

@@ -237,7 +237,7 @@ export function RenderMessageContent({
                             title={body}
                             src={src}
                             loading="lazy"
-                            style={{ objectFit: 'cover', width: '100%', height: '100%' }}
+                            style={{ objectFit: 'cover', objectPosition: 'center top', width: '100%', height: '100%' }}
                           />
                         )}
                       />

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%',
   },

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' }}
         />
       )}