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/sidebar.css

@@ -1,7 +1,8 @@
 /* ========== 侧边栏 — 对齐 zibll sidebar ========== */
+/* Width/float geometry is owned by body.site-layout-* in layout.css.
+ * Keep only non-layout chrome here. */
 .sidebar {
-  width: var(--sidebar-width, 311px);
-  flex-shrink: 0;
+  box-sizing: border-box;
 }
 
 .sidebar-inner {