Use the central Dialog for Campaign previews

webui/tests/message-preview-overlay-structure.test.mjs

@@ -15,8 +15,10 @@ assert(
   "message previews expose their responsive backdrop hook"
 );
 assert(
-  overlaySource.includes('type="button" className="modal-close" aria-label={closeLabel} title={closeLabel}'),
-  "the close control has stable button semantics, an accessible name, and a tooltip"
+  overlaySource.includes("<Dialog") &&
+    overlaySource.includes("closeLabel={closeLabel}") &&
+    overlaySource.includes('dataset.dialogStackState !== "topmost"'),
+  "message previews use the central stacked Dialog with an accessible close label"
 );
 assert(
   styles.includes("height: min(780px, calc(100dvh - 48px));"),