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>

search.php

@@ -118,7 +118,7 @@ if (is_active_sidebar('search_top_fluid')) {
 				<?php else : ?>
 					<div class="aksrt-panel search-content"><div class="text-center muted-2-color padding-20"><?php esc_html_e('未找到相关内容', 'aksrt'); ?></div></div>
 				<?php endif; ?>
-				<div class="post_ajax_loader search-loader" style="display:none"><div class="mb20"><div class="text-center muted-2-color mt20"><i class="loading mr10"></i><?php esc_html_e('加载中...', 'aksrt'); ?></div></div></div>
+				<div class="post_ajax_loader search-loader"><div class="mb20"><div class="text-center muted-2-color mt20"><i class="loading mr10"></i><?php esc_html_e('加载中...', 'aksrt'); ?></div></div></div>
 			</div>
 			<?php if (is_active_sidebar('search_bottom_content')) dynamic_sidebar('search_bottom_content'); ?>
 		</div>