fix: avoid TOC anchors being hidden by header

[OXeu]

Summary

• measure the rendered header height and expose it as a global scroll offset
• apply that offset to markdown heading anchors and TOC click scrolling
• keep anchor jumps and TOC jumps aligned across header layouts

Closes #220

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve markdown-provided heading classes

Rendering each heading with {...props} followed by a fixed className drops any class attributes coming from markdown (via rehypeRaw) or remark/rehype plugins, because the hardcoded class now overwrites props.className. This is a behavior change introduced in this commit that can break documents relying on custom heading classes for styling or JS hooks; merge props.className with the default classes instead of replacing it.

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