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/widgets/frontend.js

@@ -109,16 +109,27 @@
   }
 
   function loaderOf(container) {
+    if (!container) return null;
     return container.querySelector('.post_ajax_loader');
   }
 
+  function fallbackLoader() {
+    var text = (window.aksrtData && aksrtData.ajaxLoading) || '加载中...';
+    var node = document.createElement('div');
+    node.className = 'post_ajax_loader';
+    node.setAttribute('role', 'status');
+    node.innerHTML = '<div class="mb20"><div class="text-center muted-2-color mt20"><i class="loading mr10"></i>' + text + '</div></div>';
+    return node;
+  }
+
   function setBusy(container, busy) {
+    if (!container) return;
     if (busy) {
       container.dataset.loading = '1';
       container.setAttribute('aria-busy', 'true');
-      var loader = loaderOf(container);
       // 骨架开关走 .is-loading 类（modal-runtime.css），不写 inline display。
-      if (loader) loader.classList.add('is-loading');
+      var loader = loaderOf(container) || container.appendChild(fallbackLoader());
+      loader.classList.add('is-loading');
     } else {
       delete container.dataset.loading;
       container.setAttribute('aria-busy', 'false');
@@ -127,6 +138,21 @@
     }
   }
 
+  // 参考主题 post_ajax 的 beforeSend 语义：骨架每次请求都挪到列表末尾
+  // （最后一个条目之后、分页控件之前），请求成功后只隐藏而不删除，
+  // 下一轮翻页复用同一个节点。
+  function anchorLoader(container) {
+    var pag = container.querySelector(':scope > .ajax-pag');
+    var loader = loaderOf(container);
+    if (!loader) return null;
+    if (pag) {
+      pag.parentNode.insertBefore(loader, pag);
+    } else {
+      container.appendChild(loader);
+    }
+    return loader;
+  }
+
   function loadItems(container, page, append) {
     var owner = container.closest('[data-widget]') || container;
     var widget = owner.dataset.widget || container.dataset.widget;
@@ -134,28 +160,49 @@
     var segment = container.dataset.segment || owner.dataset.segment || '';
     if (!widget || !instance || !config.restUrl || container.dataset.loading === '1') return;
     setBusy(container, true);
-    fetch(itemsUrl(widget, instance, page, segment), { credentials: 'same-origin', headers: { Accept: 'application/json' } })
+    anchorLoader(container);
+    return fetch(itemsUrl(widget, instance, page, segment), { credentials: 'same-origin', headers: { Accept: 'application/json' } })
       .then(function (response) { if (!response.ok) throw new Error('items'); return response.json(); })
       .then(function (data) {
         var fragment = document.createElement('div');
         fragment.innerHTML = String(data.html || '');
-        Array.prototype.slice.call(container.querySelectorAll(':scope > .ajax-pag')).forEach(function (node) { node.remove(); });
         if (append) {
-          Array.prototype.slice.call(fragment.children).forEach(function (node) { container.appendChild(node); });
+          // 追加方向为底部：新条目插到骨架之前，骨架始终贴住最后一张卡片。
+          var anchor = loaderOf(container) || container.querySelector(':scope > .ajax-pag');
+          Array.prototype.slice.call(fragment.children).forEach(function (node) {
+            if (anchor && anchor.parentNode === container) {
+              container.insertBefore(node, anchor);
+            } else {
+              container.appendChild(node);
+            }
+          });
         } else {
-          container.innerHTML = fragment.innerHTML;
+          // 替换时保留骨架节点本身（参考主题 success 后只 hide() 不删除）。
+          var loader = loaderOf(container);
+          Array.prototype.slice.call(container.children).forEach(function (child) {
+            if (child !== loader && !child.classList.contains('ajax-pag')) child.remove();
+          });
+          Array.prototype.slice.call(fragment.children).forEach(function (node) { container.appendChild(node); });
         }
+        Array.prototype.slice.call(container.querySelectorAll(':scope > .ajax-pag')).forEach(function (node) { node.remove(); });
         if (data.paginate_html) container.insertAdjacentHTML('beforeend', data.paginate_html);
         container.dataset.page = String(data.page || page);
+        anchorLoader(container);
         scan(container);
       })
-      .catch(function () { container.innerHTML = errorMarkup(); })
+      .catch(function () {
+        var loader = loaderOf(container);
+        if (loader && loader.parentNode === container) {
+          loader.insertAdjacentHTML('beforebegin', errorMarkup());
+        } else {
+          container.insertAdjacentHTML('afterbegin', errorMarkup());
+        }
+      })
       .finally(function () { setBusy(container, false); });
   }
 
   function initItems(container) {
     if (!once(container, 'Items')) return;
-    if (container.classList.contains('is-loading')) loadItems(container, 1, false);
     container.addEventListener('click', function (event) {
       var retry = event.target.closest('[data-widget-retry]');
       var trigger = event.target.closest('[data-next-page]');