feat: add avatar support for agent identity

- Add avatar field to IdentityConfig type
- Add avatar parsing in AgentIdentity from IDENTITY.md
- Add renderAvatar support for image avatars in webchat
- Add CSS styling for image avatars

Users can now configure a custom avatar for the assistant in the webchat
by setting 'identity.avatar' in the agent config or adding 'Avatar: path'
to IDENTITY.md. The avatar can be served from the assets folder.

Closes #TBD

ui/src/styles/chat/grouped.css

@@ -89,6 +89,12 @@
   color: rgba(134, 142, 150, 1);
 }
 
+/* Image avatar support */
+img.chat-avatar {
+  object-fit: cover;
+  object-position: center;
+}
+
 /* Minimal Bubble Design - dynamic width based on content */
 .chat-bubble {
   position: relative;