fix(single): restore Zibll float dual-column and article chrome

Align single-post layout/visuals with Zibll: fix sidebar-over-content
float geometry (position:unset, 311px token, kill Bootstrap/container
padding and the 280px sidebar override), demote theme-box to spacing-
only, unify breadcrumb/cover/graphic utilities, and split aksrt-single
render helpers under the file-size limit.

assets/css/frontend/sidebar.css

@@ -156,13 +156,8 @@
   margin-bottom: 12px;
 }
 
-/* 侧边栏响应式 */
-@media (max-width: 1200px) {
-  .sidebar {
-    width: 280px;
-  }
-}
-
+/* 侧边栏响应式 — width/float 由 layout.css 的 --sidebar-width + site-layout-* 拥有。
+ * 禁止在这里硬编码 280px（会与 content-layout gutter 失配导致叠层/空隙）。 */
 @media (max-width: 991px) {
   .sidebar {
     display: none;