ui: generous whitespace — floating compose, spacious messages + header

- Compose: floating bordered container with inner padding, no top border
- Compose: bottom bar with prefix + "enter to send" hint
- Messages: increased padding (py-3/4, px-4/5)
- Header: more breathing room (py-3/4, px-4/6)
- Modern 2026 spacing throughout

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

ui/colony/src/components/MessageItem.tsx

@@ -66,7 +66,7 @@ export function MessageItem({ message, replyTarget, onReply }: Props) {
         </div>
       )}
 
-      <div className="px-4 py-2">
+      <div className="px-4 py-3 md:px-5 md:py-4">
         {/* Header */}
         <div className="flex items-center gap-2 text-[11px] flex-wrap">
           {/* Name */}