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.

tests/widget-system/post-visual-contract.test.cjs

@@ -689,7 +689,6 @@ test('single articles enqueue only content and settings driven assets', () => {
   assert.doesNotMatch(singleBlock, /\$enqueue_module\('single-post'\)|\$enqueue_module\('comments'\)/);
 
   for (const requirement of [
-    'has_code',
     'needs_navigation',
     'needs_cover',
     'needs_related_toc',
@@ -707,7 +706,6 @@ test('single articles enqueue only content and settings driven assets', () => {
     assert.match(singleBlock, new RegExp(`singular_assets\\['${requirement}'\\]`));
   }
 
-  assert.match(runtime, /wp:\(\?:code\|preformatted\|jio\\\/enlighter\)/);
   assert.match(runtime, /\$requirements\['has_toc'\] = \$is_article && \$article_nav_enabled/);
   assert.match(runtime, /data-jio-video/);
   assert.match(runtime, /post_password_required\(\$post_id\)/);
@@ -718,7 +716,6 @@ test('single articles enqueue only content and settings driven assets', () => {
   assert.match(runtime, /needs_share'[\s\S]*?share_enabled/);
 
   for (const requirement of [
-    'has_code',
     'has_toc',
     'has_video',
     'needs_reward',