fix(pagination): align AJAX load-more loading state with reference

Loading indication now matches the reference theme's post_ajax() contract:

- Stop overwriting the 加载更多 button label with 正在加载...; the button
  keeps its own text and the .post_ajax_loader skeleton carries the busy
  state instead.
- Add the reference fallback loader (.loading ring + 加载中...) when no
  skeleton exists in the DOM, and reinsert the skeleton after the last
  card on every page load like beforeSend does.
- Retext the loader string to 加载中... (zib__('loading')) and make the
  ajax_trigger default a translatable string.
- Route ajax-list.js / comments.js / widgets frontend.js through the
  .is-loading class instead of inline display, which outranked the class
  rule and could freeze the skeleton shut.
- Drop the search.php inline display:none that defeated .is-loading, and
  remove the unproduced .post-ajax-loader / [data-aksrt-ajax-loader] shims.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/widgets/frontend.js

@@ -117,12 +117,13 @@
       container.dataset.loading = '1';
       container.setAttribute('aria-busy', 'true');
       var loader = loaderOf(container);
-      if (loader) loader.style.display = '';
+      // 骨架开关走 .is-loading 类（modal-runtime.css），不写 inline display。
+      if (loader) loader.classList.add('is-loading');
     } else {
       delete container.dataset.loading;
       container.setAttribute('aria-busy', 'false');
       var loader = loaderOf(container);
-      if (loader) loader.style.display = 'none';
+      if (loader) loader.classList.remove('is-loading');
     }
   }