fix compact message bottom padding — match group padding on last message

The last compact message in a group had py-0.5, leaving too little
space before the border/next group. Now uses pb-3/pb-4 when lastInGroup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/colony/src/components/MessageItem.tsx

@@ -115,7 +115,7 @@ export function MessageItem({ message, compact, lastInGroup, replyTarget, onSele
         </button>
       )}
 
-      <div className={cn("px-4 md:px-5", compact ? "py-0.5" : "py-3 md:py-4")}>
+      <div className={cn("px-4 md:px-5", compact ? (lastInGroup ? "pt-0.5 pb-3 md:pb-4" : "py-0.5") : "py-3 md:py-4")}>
         {/* Header — hidden in compact mode */}
         {!compact && <div className="flex items-center gap-2.5 text-[11px] flex-wrap">
           {/* Avatar — ape emoji with OKLCH color, agents get first letter */}