match folded activity-card spacing to the conversation cards (T-282)

The collapsed activity / agent-run cards (ClideHolderCard) wrapped themselves
in 3px vertical margin, while the prose ConversationCards use 14px bottom /
0 top. So a folded card floated ~17px below the previous card but hugged the
next one at 3px — the uneven gap the earlier bordered-padding tweak didn't
address. Give the holder the same bottom-14 / top-0 margin.

The holder-card golden is regenerated for the taller frame. The copy-button
holder test parks its hover and advances past the tooltip show-delay so the
(exit-uncancellable) Future.delayed timer fires instead of leaking.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/builtin/claude/holder_card_test.dart

@@ -114,6 +114,12 @@ void main() {
     expect(f.services.clipboard.readAs<String>(), 'copied from a held card');
     // And the holder stayed expanded (the copy tap wasn't a background toggle).
     expect(find.bySemanticsLabel('Activity, 2 steps, expanded'), findsOneWidget);
+
+    // The hover starts a ClideTooltip show-delay (a Future.delayed that exit
+    // can't cancel). Move off and advance past the delay so it fires and leaves
+    // no pending timer at teardown.
+    await g.moveTo(const Offset(-100, -100));
+    await tester.pump(const Duration(milliseconds: 600));
   });
 
   testWidgets('the explicit control is keyboard-focusable and toggles on Activate (a11y)', (tester) async {