fix(single): align article code blocks with the reference theme

The article body ran two highlighting pipelines at once: the runtime
EnlighterJS pass (same one the comment area uses) plus a bespoke
precode.js enhancer, and a hand-rolled .code-block skin. The two
disagreed, so rendered blocks carried a dark #282c34 card with 15px
padding, 14px text, a border and a second copy button stacked on top of
the EnlighterJS toolbar.

Drop the parallel path and let one pipeline own code blocks:
- delete precode.js, single-post/code-blocks.css and the unused
  assets/js/enlighter copy that only precode.js referenced
- drop the now-dead has_code requirement (PHP scan + feature enqueue)
- render pre exactly as the reference does: bootstrap's pre box
  (9.5px padding / 13px / radius 4) plus main.css's muted background,
  with the EnlighterJS theme supplying the skin

Also scope the inline-code skin to :not(pre code) so it stops painting
the block background pink, and remap the toolbar copy glyph: \f0ea is
the clipboard in FA4 but "paste" in FA5; the clipboard is \f328.

Verified against the reference at 1440px both themes: pre 13px/18.57,
9.5px padding, radius 4, muted background; EnlighterJS block heights
151.31 / 60.38 match exactly.

assets/css/frontend/single-post.css

@@ -1,7 +1,6 @@
 @import url("legacy-utilities.css");
 @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");
 /* single-post/cover.css 不经 @import 引入：@import 无版本号，浏览器会拿
  * 旧缓存（含已删除的 .single-cover padding 规则）把封面高度链挤成 0。