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/server/package.json

@@ -23,7 +23,8 @@
     "drizzle-orm": "^0.33.0",
     "fastify": "^4.28.1",
     "jsonwebtoken": "^9.0.2",
-    "livekit-server-sdk": "^2.6.1"
+    "livekit-server-sdk": "^2.6.1",
+    "sharp": "^0.33.0"
   },
   "devDependencies": {
     "@types/bcryptjs": "^2.4.6",