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/code-blocks.css

@@ -1,128 +0,0 @@
-/* ========== 代码块：GitHub 风格 ========== */
-.code-block {
-  border-radius: var(--radius-code);
-  border: 1px solid var(--color-line);
-  overflow: hidden;
-  margin: 1.5em 0;
-  background: var(--surface-3);
-}
-
-.code-block__head {
-  display: flex;
-  align-items: center;
-  justify-content: space-between;
-  height: 36px;
-  padding: 0 12px;
-  background: var(--surface-3);
-  border-bottom: 1px solid var(--color-line);
-  font-size: 12px;
-  color: var(--color-ink-3);
-}
-
-.code-block__copy {
-  display: inline-flex;
-  align-items: center;
-  gap: 6px;
-  padding: 6px 12px;
-  border-radius: 6px;
-  border: 1px solid var(--color-line);
-  background: var(--surface-page);
-  cursor: pointer;
-  font-size: 12px;
-  color: var(--color-ink-3);
-}
-
-.code-block__copy:hover {
-  border-color: var(--color-primary);
-  color: var(--color-primary);
-}
-
-.code-block__body {
-  overflow-x: auto;
-  padding: 16px 20px;
-  font-family: var(--font-mono);
-  font-size: 13.5px;
-  line-height: 1.6;
-}
-
-@media (max-width: 767px) {
-  .code-block__body {
-    font-size: 12px;
-    padding: 12px 14px;
-  }
-}
-
-:is(.entry-content, .wp-posts-content) pre.wp-block-code,
-:is(.entry-content, .wp-posts-content) pre:not(.wp-block-preformatted) {
-  border-radius: var(--radius-code);
-  border: 1px solid var(--color-line);
-  background: var(--surface-3);
-  padding: 0;
-  overflow: hidden;
-}
-
-:is(.entry-content, .wp-posts-content) pre code {
-  display: block;
-  padding: 16px 20px;
-  overflow-x: auto;
-  background: transparent;
-  font-size: 13.5px;
-  line-height: 1.6;
-}
-
-/* Syntax highlight utilities (plugin or wrapped markup) */
-.gh-keyword {
-  color: #cf222e;
-}
-
-html.dark .gh-keyword,
-html.dark .gh-keyword {
-  color: #ff7b72;
-}
-
-.gh-string {
-  color: #0a3069;
-}
-
-html.dark .gh-string,
-html.dark .gh-string {
-  color: #a5d6ff;
-}
-
-.gh-comment {
-  color: #6e7781;
-  font-style: italic;
-}
-
-html.dark .gh-comment,
-html.dark .gh-comment {
-  color: #8b949e;
-}
-
-.gh-function {
-  color: #8250df;
-}
-
-html.dark .gh-function,
-html.dark .gh-function {
-  color: #d2a8ff;
-}
-
-.gh-var {
-  color: #0550ae;
-}
-
-html.dark .gh-var,
-html.dark .gh-var {
-  color: #79c0ff;
-}
-
-.gh-op {
-  color: #24292f;
-}
-
-html.dark .gh-op,
-html.dark .gh-op {
-  color: #c9d1d9;
-}
-

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

@@ -205,8 +205,10 @@ article.article.box-body,
   margin: 1.5em 0;
 }
 
-/* 行内代码 — 对齐 legacy: 粉色文字 + 粉色背景 */
-:is(.entry-content, .wp-posts-content) code {
+/* 行内代码 — 对齐 legacy: 粉色文字 + 粉色背景。
+ * 必须排除 pre code：代码块底色/字号由 bootstrap 的 pre 与 EnlighterJS
+ * 主题决定，行内皮肤套到块内会把整块代码染成粉底。 */
+:is(.entry-content, .wp-posts-content) code:not(pre code) {
   font-family: var(--font-mono);
   font-size: 0.9em;
   color: #ff3c98;