feat(frontend): lazy-load images, add lightbox viewer, polish AI panel

- Add loading=lazy + decoding=async to all <img> (message card preview,
  attachments grid, image grid, avatars via ui/avatar)
- New Lightbox component: fullscreen image viewer with keyboard nav
  (←/→/Esc), counter, click-to-close; wired into messages page + detail
- Attachments grid: click image to open, image counter badge, grouped
  non-image attachments
- AI analysis panel: line-clamp-3 with Show more/less toggle
- Message card: preview image is now a clickable button opening detail

services/frontend/src/components/ui/avatar.tsx

@@ -33,6 +33,8 @@ function AvatarImage({ className, ...props }: AvatarPrimitive.Image.Props) {
         "aspect-square size-full rounded-full object-cover",
         className,
       )}
+      loading="lazy"
+      decoding="async"
       {...props}
     />
   );