fix(chat): soften final answer divider and tighten activity spacing

The final answer divider was too prominent and sat too far below Activity. Lower its opacity to 30% and remove the header's bottom margin in both states, matching the maintainer's visual adjustments.

Validated UI type-check, lint for both changed files, and seven existing activity and rendering tests.

packages/ui/src/components/chat/components/LiveTurnActivity.tsx

@@ -67,7 +67,7 @@ export function LiveTurnActivity({ turn, hasLaterAssistant, expanded, onToggle,
         <div className="relative z-0" data-live-turn-activity={turn.turnId}>
             {settled ? (
                 <div className="chat-message-column @container">
-                    <div className="mt-1 mb-2">
+                    <div className="mt-1">
                         <Button variant="ghost" size="sm" className={headerClass} onClick={onToggle}
                             aria-expanded={isExpanded} aria-controls={finalMessage ? `${contentId} ${finalContentId}` : contentId}>
                             {label}

packages/ui/src/components/chat/message/MessageBody.tsx

@@ -1843,7 +1843,7 @@ const AssistantMessageBody = React.memo(({
                         <div
                             key={`final-answer-divider-${messageId}`}
                             aria-hidden="true"
-                            className="mt-1.5 mb-4 h-px w-full bg-muted-foreground/60"
+                            className="mt-1.5 mb-4 h-px w-full bg-muted-foreground/30"
                         />
                     );
                 }