feat(frontend): adopt zibll site-layout-1/2/3 float dual-column system

Wire settings-driven sidebar visibility and body site-layout classes,
replace content-layout CSS grid with Zibll float geometry, move sidebar
DOM to content-wrap sibling across single/archive/page/home, and add
post meta show_layout overrides.

assets/css/frontend/layout.css

@@ -109,62 +109,122 @@
   font-weight: 600;
 }
 
-.content-layout {
-  display: grid;
-  grid-template-columns: 1fr;
-  gap: 20px;
-  align-items: start;
+/* ============================================================
+ * Site shell — Zibll site-layout-1/2/3 float dual-column
+ * body.site-layout-1 = no sidebar
+ * body.site-layout-2 = sidebar right
+ * body.site-layout-3 = sidebar left
+ * ============================================================ */
+
+main.container,
+main.content-shell,
+.site-main.container {
+  position: relative;
 }
 
-@media (min-width: 992px) {
-  .content-layout {
-    grid-template-columns: minmax(0, 1fr) var(--sidebar-width, 311px);
-    gap: 20px;
-  }
+main.container::after,
+main.content-shell::after,
+.site-main.container::after {
+  content: "";
+  display: table;
+  clear: both;
+}
+
+.content-wrap {
+  float: left;
+  width: 100%;
+  min-width: 0;
+}
+
+.content-layout {
+  display: block;
+  width: 100%;
+  min-width: 0;
+  margin-right: 0;
+  margin-left: 0;
 }
 
 .content-layout__primary {
   min-width: 0;
+  max-width: 100%;
 }
 
-.content-layout__aside {
+.content-layout__aside,
+.sidebar {
   min-width: 0;
 }
 
+/* No sidebar */
+body.site-layout-1 .sidebar {
+  display: none;
+}
+
+body.site-layout-1 .content-layout,
+body.site-layout-1 .content-layout--no-sidebar {
+  margin-left: 0;
+  margin-right: 0;
+}
+
+/* Compat: templates may still stamp content-layout--no-sidebar */
+.content-layout--no-sidebar {
+  margin-left: 0 !important;
+  margin-right: 0 !important;
+}
+
+/* Sidebar RIGHT (layout 2) — Zibll default */
+body.site-layout-2 .content-layout {
+  margin-right: calc(var(--sidebar-width, 311px) + 15px);
+  margin-left: 0;
+}
+
+body.site-layout-2 .sidebar {
+  float: left;
+  width: var(--sidebar-width, 311px);
+  margin-left: calc(0px - var(--sidebar-width, 311px));
+  position: relative;
+}
+
+/* Sidebar LEFT (layout 3) */
+body.site-layout-3 .content-layout {
+  margin-left: calc(var(--sidebar-width, 311px) + 15px);
+  margin-right: unset;
+}
+
+body.site-layout-3 .sidebar {
+  float: left;
+  width: var(--sidebar-width, 311px);
+  margin-left: -100%;
+  position: relative;
+}
+
+/* Sticky widgets inside sidebar */
 @media (min-width: 992px) {
+  body.site-layout-2 .sidebar .posts-nav-box,
+  body.site-layout-3 .sidebar .posts-nav-box,
+  .sidebar .content-layout__aside,
   .content-layout__aside {
     position: sticky;
     top: calc(var(--nav-height, 60px) + 16px);
-    align-self: start;
   }
 
-  /* WordPress Admin Bar 已全局禁用，无需偏移规则 */
-}
-
-@media (min-width: 992px) {
+  .has-static-nav .sidebar .posts-nav-box,
   .has-static-nav .content-layout__aside {
     top: 16px;
   }
 }
 
-.content-layout--no-sidebar {
-  grid-template-columns: 1fr;
-}
-
-.content-layout--no-sidebar .content-layout__primary {
-  width: 100%;
-  max-width: var(--content-no-sidebar-max, 1080px);
-  margin: 0 auto;
-}
-
-/* 三栏布局变体：侧边栏在左侧 */
-@media (min-width: 992px) {
-  .content-layout--sidebar-left {
-    grid-template-columns: var(--sidebar-width, 311px) minmax(0, 1fr);
+/* Mobile collapse (Zibll max-width: 991px) */
+@media (max-width: 991.98px) {
+  body.site-layout-2 .content-layout,
+  body.site-layout-3 .content-layout {
+    margin-left: auto;
+    margin-right: auto;
   }
 
-  .content-layout--sidebar-left .content-layout__aside {
-    order: -1;
+  body.site-layout-2 .sidebar,
+  body.site-layout-3 .sidebar,
+  .sidebar {
+    display: none;
   }
 }