Custom icon styling tweaks (#1877)

Co-authored-by: Garrick Aden-Buie <garrick@posit.co>

Author: cpsievert

js/chat/chat.scss

@@ -83,6 +83,8 @@ shiny-chat-message {
       // images and avatars are full-bleed
       height: 100%;
       width: 100%;
+      max-width: 100%;
+      max-height: 100%;
       margin: 0 !important;
       object-fit: contain;
     }
@@ -95,6 +97,13 @@ shiny-chat-message {
       max-height: 66%;
       max-width: 66%;
     }
+
+    // Provide .border-0 as a way to opt-out of border container
+    &:has(> .border-0) {
+      border: none;
+      border-radius: unset;
+      overflow: unset;
+    }
   }
 
   /* Vertically center the 2nd column (message content) */