fix(ui): allow relative URLs in avatar validation

The isAvatarUrl check only accepted http://, https://, or data: URLs,
but the /avatar/{agentId} endpoint returns relative paths like /avatar/main.
This caused local file avatars to display as text instead of images.

Fixes avatar display for locally configured avatar files.

ui/src/ui/chat/grouped-render.ts

@@ -158,7 +158,8 @@ function renderAvatar(
 function isAvatarUrl(value: string): boolean {
   return (
     /^https?:\/\//i.test(value) ||
-    /^data:image\//i.test(value)
+    /^data:image\//i.test(value) ||
+    /^\//.test(value) // Relative paths from avatar endpoint
   );
 }