fix(style): move aside/TOC breakpoint override to _head_extend.html

Avoid modifying the serene submodule directly; apply the 1400px
breakpoint for hiding aside and #blank via project-level CSS override.

Author: lemonadern

templates/_head_extend.html

@@ -225,6 +225,11 @@
       grid-template-columns: 1fr;
     }
 
+    body.post aside,
+    body.post #blank {
+      display: none;
+    }
+
     body.post aside nav {
       display: none;
     }