fix(single): close remaining live gaps vs Zibll demo

Align runtime --sidebar-width (300→311), --main-radius, and
body.nav-fixed 88px reserve; enqueue single-post CSS parts directly;
match Zibll action buttons and author avatar DOM; tighten dual-column
tokens after comparing blog.rutua.cn with demo.zibll.com.

assets/css/frontend/nav/base.css

@@ -1,6 +1,19 @@
-/* Zibll body.nav-fixed: reserve header space then pull hero under nav */
+/* Zibll body.nav-fixed: hard 88px reserve (main.css), not bare --nav-height.
+ * Nav bar itself is ~60px; extra space matches zibll chrome/shadow gap. */
 body.nav-fixed {
-  padding-top: var(--nav-height, 60px);
+  padding-top: 88px;
+}
+
+@media (max-width: 768px) {
+  body.nav-fixed {
+    padding-top: 72px;
+  }
+}
+
+@media (max-width: 640px) {
+  body.nav-fixed {
+    padding-top: 60px;
+  }
 }
 
 /* Fixed nav without hero overlay: take header out of flow.