feat(frontend): align single post page 1:1 with Zibll

Restore Zibll float dual-column shell, fix single settings DTO key mapping,
reorder article footer/actions, and match author bar, prev/next, title/body
typography, and limit-height expand UX.

assets/css/frontend/single-post/shell.css

@@ -0,0 +1,59 @@
+/* Single page shell — Zibll float dual-column (scoped to body.single) */
+
+body.single .container {
+  position: relative;
+}
+
+body.single .content-wrap {
+  float: left;
+  width: 100%;
+}
+
+/* Clear floats after main dual-column shell */
+body.single main.container::after {
+  content: "";
+  display: table;
+  clear: both;
+}
+
+/* Default: no sidebar full width */
+body.single.container--no-sidebar .content-layout,
+body.single .container--no-sidebar .content-layout,
+body.single main.container--no-sidebar .content-layout {
+  margin-right: 0;
+}
+
+/* Zibll site-layout-2: content left, sidebar right (311px) */
+@media (min-width: 992px) {
+  body.single:not(.no-sidebar) main.container:not(.container--no-sidebar) .content-layout {
+    margin-right: calc(var(--sidebar-width, 311px) + 15px);
+  }
+
+  body.single:not(.no-sidebar) main.container:not(.container--no-sidebar) > .sidebar {
+    float: left;
+    width: var(--sidebar-width, 311px);
+    margin-left: calc(0px - var(--sidebar-width, 311px));
+    position: relative;
+  }
+}
+
+@media (max-width: 991.98px) {
+  body.single main.container > .sidebar {
+    display: none;
+  }
+
+  body.single main.container .content-layout {
+    margin-right: 0 !important;
+  }
+}
+
+/* Sidebar widgets spacing */
+body.single .sidebar .zib-widget,
+body.single .sidebar .widget {
+  margin-bottom: 15px;
+}
+
+body.single .sidebar .posts-nav-box.toc-widget {
+  position: sticky;
+  top: calc(var(--nav-height, 60px) + 16px);
+}