feat: image optimization — client-side resize + server-side thumbnails

Avatars/banners now resize to max 512px/1920px and convert to WebP before
upload (zero server cost). Chat image uploads generate an 800px-wide WebP
thumbnail via Sharp; the feed shows the thumbnail, click opens the full-res
original. Adds lazy loading to avatars. Federation-compatible: remote
instances without this feature fall back gracefully.

packages/web/src/components/ui/Avatar.tsx

@@ -105,6 +105,7 @@ export function Avatar({ src, name, size = 40, status, className = '', onClick,
             src={(src.startsWith('http') || src.startsWith('blob:') || src.startsWith('data:'))
               ? src : `/api/uploads/${src}`}
             alt={name}
+            loading="lazy"
             className="w-full h-full rounded-full object-cover"
             onError={(e) => {
               (e.target as HTMLImageElement).style.display = 'none';