Add syntax highlighting and light/dark themes to twui CodeBlock

- Highlight string children with refractor (Prism) when `language` is
  set, rendered as React token spans; add ini/bash/json/yaml/ts support
- Add `theme` prop (auto | light | dark); auto follows the `.dark` class.
  Palettes are `--twui-code-*` CSS variables (GitHub Light/Dark, all
  tokens WCAG AA) so custom themes need no component changes
- Fix layout: real padding, singleBlock copy button no longer covers
  the code, clipped rounded corners, wrapperProps.style honoured,
  long tokens wrap
- MDX helpers take `codeTheme` and no longer force a black background
- Highlight WireGuard config blocks as ini

Co-Authored-By: Claude Opus 5.5 <[email protected]>

package.json

@@ -39,6 +39,7 @@
         "openai": "^7.4.0",
         "qrcode": "^1.5.4",
         "react-markdown": "^10.1.0",
+        "refractor": "^5.0.0",
         "rehype-prism-plus": "^2.0.2",
         "remark-gfm": "^4.0.1",
         "sharp": "^0.35.3",