fix(ui): add vertical margin to composer handoff preview row (PAP-11281)

The 'No agent will be notified' / 'Wake' preview row sat flush against the
'Did you mean' mention coach box above it and the Send button row below it.
Wrap it in a my-2 spacer and skip the wrapper when the preview is empty so no
phantom margin is rendered.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ui/src/components/IssueChatThread.tsx

@@ -3840,8 +3840,10 @@ const IssueChatComposer = forwardRef<IssueChatComposerHandle, IssueChatComposerP
         </div>
       ) : null}
 
-      {body.trim() ? (
-        <ComposerHandoffPreviewRow preview={handoffPreview} resolvers={handoffResolvers} />
+      {body.trim() && handoffPreview.kind !== "none" ? (
+        <div className="my-2">
+          <ComposerHandoffPreviewRow preview={handoffPreview} resolvers={handoffResolvers} />
+        </div>
       ) : null}
 
       <div className="flex flex-wrap items-center justify-end gap-3">