fix(ajax): show the reference skeleton on every load-more surface

Clicking 加载更多 in the 多栏目文章 widget showed no skeleton at all — the
list simply stayed still until the response arrived, then the new cards
appeared. The homepage stream, archives and comments already had a working
skeleton; the widget's AJAX load goes through a different client and never
found its loader.

Root cause was DOM placement, not visibility: ContentRenderer emitted
.post_ajax_loader as a *sibling* of .widget-ajaxpager, while the widget REST
client resolves it with container.querySelector('.post_ajax_loader'). The
node existed but could never match, so .is-loading was never added and the
skeleton stayed display:none. Inactive tab panes had the same shape, and the
direct-load branch of ajaxpager() emitted no loader at all.

Changes, all following the reference theme's post_ajax() contract in
zibll/js/main.js (beforeSend anchors the loader to the end of the list;
success only hides it, never removes it):

- ContentRenderer: nest the loader inside .widget-ajaxpager for both the
  direct-load and inactive-tab branches, and drop the sibling append.
- widgets/frontend.js: anchorLoader() moves the loader to the end of the
  list before .ajax-pag on every request; appended cards are inserted before
  it; a replace preserves the node instead of wiping it with innerHTML; a
  fallback loader is created when the DOM has none; loadItems() now returns
  its promise; drop the unreachable is-loading auto-load branch.
- Message centre, user-home panels and the search page switch from spinner /
  opacity feedback to the same .posts-item-shaped skeleton, so all AJAX
  surfaces share one loading contract.
- Remove the second, never-wired skeleton system (.aksrt-skeleton-* in
  core-ui.css) and the dead spinner stylesheets/rules (skeleton.css,
  ajax-spinner.css, .load-more__*, --skeleton-base/--skeleton-shine) plus the
  unregistered assets/js/waterfall.js.
- Extend the widget, message-centre, user-home and asset-ledger contract
  tests to pin the loader inside the container, the class-only visibility
  switch, and the absence of the deleted dead code.

Verified: 470 node tests, 136 PHP widget assertions, admin-contract and
widget-system audits all green; rebuilt public assets after the CSS/JS
removals.

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

assets/js/messages.js

@@ -70,11 +70,18 @@
     currentList.querySelectorAll('[data-notification-id]').forEach(function (item) {
       existingIds.add(item.dataset.notificationId);
     });
+    // 骨架常驻列表末尾：新条目一律插到它之前，形状与参考主题一致
+    // （post_ajax 的 beforeSend 把 loader 挪到最后一张卡片之后）。
+    var skeleton = currentList.querySelector(':scope > .post_ajax_loader');
     var firstAdded = null;
     incomingList.querySelectorAll('.message-item').forEach(function (item) {
       if (existingIds.has(item.dataset.notificationId)) return;
       if (!firstAdded) firstAdded = item;
-      currentList.appendChild(item);
+      if (skeleton) {
+        currentList.insertBefore(item, skeleton);
+      } else {
+        currentList.appendChild(item);
+      }
     });
 
     var currentPagination = content.querySelector('.messages-pagination');
@@ -93,6 +100,9 @@
     var settings = options || {};
     root.classList.add('is-loading');
     content.setAttribute('aria-busy', 'true');
+    // 骨架只在 AJAX 请求期间展开（.is-loading），与文章列表翻页同一契约。
+    var skeleton = content.querySelector('.messages-list > .post_ajax_loader');
+    if (skeleton) skeleton.classList.add('is-loading');
 
     return post('aksrt_load_message_center', {
       view: requested.view,
@@ -111,7 +121,19 @@
       if (settings.append && state.paginateType === 'ajax_lists') {
         firstAdded = appendMessagePage(result.data.content);
       } else {
+        // 替换时保留骨架容器（参考主题 success 后只 hide() 不删除），
+        // 让后续翻页复用同一节点而不是回到无骨架状态。
+        var keep = content.querySelector('.post_ajax_loader');
         content.innerHTML = result.data.content;
+        var nextSkeleton = content.querySelector('.post_ajax_loader');
+        if (keep && !nextSkeleton) {
+          var list = content.querySelector('.messages-list');
+          if (list) {
+            list.appendChild(keep);
+          } else {
+            content.appendChild(keep);
+          }
+        }
       }
       updateHeaderBadge(result.data.unread);
 
@@ -131,6 +153,8 @@
     }).finally(function () {
       root.classList.remove('is-loading');
       content.removeAttribute('aria-busy');
+      var loader = content.querySelector('.messages-list > .post_ajax_loader');
+      if (loader) loader.classList.remove('is-loading');
     });
   }