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/layout.css

@@ -1,63 +1,4 @@
-/* ========== 面包屑 — 对齐 zibll .breadcrumb ========== */
-.breadcrumbs-wrap {
-  margin-bottom: 18px;
-}
-
-.breadcrumbs {
-  display: flex;
-  align-items: center;
-  flex-wrap: wrap;
-  gap: 0;
-  margin: 0;
-  padding: 0;
-  list-style: none;
-  font-size: 13px;
-  color: var(--muted-2-color, #999);
-  opacity: 0.8;
-}
-
-.breadcrumbs__item {
-  display: inline-flex;
-  align-items: center;
-  min-width: 0;
-}
-
-.breadcrumbs__item a,
-.breadcrumbs__item span {
-  display: inline-flex;
-  align-items: center;
-  gap: 6px;
-}
-
-.breadcrumbs a {
-  color: var(--muted-2-color, #999);
-}
-
-.breadcrumbs a:hover {
-  color: var(--focus-color, #f04494);
-}
-
-.breadcrumbs__sep {
-  margin: 0 0 0 6px;
-  opacity: 0.6;
-}
-
-@media (max-width: 767px) {
-  .breadcrumbs {
-    font-size: 12px;
-  }
-
-  .breadcrumbs-wrap {
-    margin-bottom: 14px;
-  }
-}
-
-/* 封面内面包屑 */
-.single-cover-con .breadcrumbs {
-  font-size: 13px;
-  margin: 5px 0 0 -6px;
-  opacity: unset;
-}
+/* Breadcrumb chrome lives in breadcrumb.css (single source). */
 
 /* ========== 主布局 ========== */
 .content-main {
@@ -149,8 +90,16 @@ main.content-shell::after,
   max-width: 100%;
 }
 
-.content-layout__aside,
+/* Base sidebar column (Zibll .sidebar defaults) — layout variants override below */
 .sidebar {
+  float: left;
+  width: var(--sidebar-width, 311px);
+  min-width: 0;
+  position: relative;
+  box-sizing: border-box;
+}
+
+.content-layout__aside {
   min-width: 0;
 }
 
@@ -171,7 +120,7 @@ body.site-layout-1 .content-layout--no-sidebar {
   margin-right: 0 !important;
 }
 
-/* Sidebar RIGHT (layout 2) — Zibll default */
+/* Sidebar RIGHT (layout 2) — Zibll main.css 1:1 */
 body.site-layout-2 .content-layout {
   margin-right: calc(var(--sidebar-width, 311px) + 15px);
   margin-left: 0;
@@ -181,10 +130,11 @@ body.site-layout-2 .sidebar {
   float: left;
   width: var(--sidebar-width, 311px);
   margin-left: calc(0px - var(--sidebar-width, 311px));
-  position: relative;
+  /* Zibll uses position:unset so sticky children aren't trapped */
+  position: unset;
 }
 
-/* Sidebar LEFT (layout 3) */
+/* Sidebar LEFT (layout 3) — Zibll main.css 1:1 */
 body.site-layout-3 .content-layout {
   margin-left: calc(var(--sidebar-width, 311px) + 15px);
   margin-right: unset;
@@ -194,7 +144,7 @@ body.site-layout-3 .sidebar {
   float: left;
   width: var(--sidebar-width, 311px);
   margin-left: -100%;
-  position: relative;
+  position: unset;
 }
 
 /* Sticky widgets inside sidebar */