fix(desktop): increase media thumbnail height from 160px to 288px

Change from h-40 object-cover (fixed crop) to max-h-72 object-contain
so images display at proper aspect ratio without cropping.

ui/desktop/frontend/src/components/chat/MediaBlock.tsx

@@ -48,7 +48,7 @@ export function MediaBlock({ items }: MediaBlockProps) {
               onClick={() => setLightboxIndex(i)}
               className="group relative overflow-hidden rounded-lg border border-border cursor-pointer"
             >
-              <AuthImage src={item.url} alt="" className="h-40 w-full object-cover" />
+              <AuthImage src={item.url} alt="" className="max-h-72 w-full object-contain" />
               <div className="pointer-events-none absolute inset-0 bg-gradient-to-t from-black/40 via-transparent to-transparent opacity-0 transition-opacity group-hover:opacity-100" />
             </button>
           ))}