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>

CHANGELOG.md

@@ -201,6 +201,9 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- Folded activity and agent-run cards in the Claude conversation now use the
+  same bottom spacing as the prose cards around them, instead of sitting
+  cramped 3px below the next card. (T-282)
 - The welcome screen no longer overflows on a short or narrow window — its
   content scrolls when it can't fit and stays centred when it can, and a long
   git-branch name on a recent-project row now truncates with an ellipsis. (T-273)