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.css

@@ -1,3 +1,4 @@
+@import url("single-post/shell.css");
 @import url("single-post/content.css");
 @import url("single-post/code-blocks.css");
 @import url("single-post/navigation.css");