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/shared/src/types.ts

@@ -209,6 +209,7 @@ export interface Attachment {
   originalName: string;
   mimetype: string;
   size: number;
+  thumbnailFilename?: string | null;
   createdAt: number;
 }