fix(posts): repair the load-more skeleton and align it with the reference

Clicking 加载更多 showed no skeleton at all, and the placeholder shape did
not match the reference theme's post cards.

- Class-name mismatch: pagination.js queried .post-ajax-loader while every
  PHP producer emits .post_ajax_loader, so showSkeleton() never found a
  target and the loader stayed hidden. Accept both spellings.
- Inline display:none on the loader (pagination, comments, widget tab
  panes) outranked .is-loading, so even the correct selector could not
  reveal it. Drop the inline style; the reference uses a bare
  <div class="post_ajax_loader"> and toggles visibility from JS.
- loadNextPage() set indicator.style.display = "flex", which forced the
  .posts-item.card skeleton blocks (inline-block layout) into a single
  row instead of stacking vertically — the visible symptom. Remove those
  inline writes and drive .is-loading from CSS for every surface.
- Scope the hide/show rules on the class itself rather than requiring an
  .ajaxpager ancestor: homepage tab panes, archive streams and comment
  lists sit at three different depths.
- Rebuild aksrt_render_placeholder() on zib_get_post_placeholder's
  structure — list = t1/k1/k2 + two s1, card = t1 item-heading/k2 + two
  s1, thumb wrapped in post-graphic + radius8 item-thumbnail, all carrying
  ajax-item so the loader is not mistaken for a real card.
- Add mini/mini_thumb modes so ContentRenderer::placeholder() stops
  hand-rolling the same markup; normalize its 'lists' arg to 'list'.

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

assets/css/frontend/modal-runtime.css

@@ -257,13 +257,16 @@ body.aksrt-modal-runtime-open {
   min-height: 80px;
 }
 
-.ajaxpager .post_ajax_loader,
-.ajaxpager .post-ajax-loader {
+/* 骨架默认收起，由 pagination.js 的 showSkeleton() 加 .is-loading 展开。
+   这里不依赖 .ajaxpager 祖先：首页 tab pane、归档流、评论列表的 loader
+   层级各不相同，统一按类名自身控制。 */
+.post_ajax_loader,
+.post-ajax-loader {
   display: none;
 }
 
-.ajaxpager .post_ajax_loader.is-loading,
-.ajaxpager .post-ajax-loader.is-loading {
+.post_ajax_loader.is-loading,
+.post-ajax-loader.is-loading {
   display: block;
 }