Fix broken footnote navigation caused by rehype-sanitize doubling the
`user-content-` ID prefix (remark-gfm already generates prefixed IDs).
Set `clobberPrefix: ""` in the sanitize schema to prevent the clash.

Add smooth scroll behavior to all internal hash links (#-prefixed),
so clicking a footnote reference smoothly scrolls to its definition
and the backlink (↩) scrolls back to the reference.

Closes #155

Preserve browser back-button navigation through footnote/heading links,
matching native anchor link behavior.

- Preserve rehype-sanitize DOM-clobbering protection by keeping the
  default clobberPrefix and adding a rehype plugin that strips the
  user-content- prefix from remark-gfm footnote IDs before sanitize
  re-adds it (avoiding double-prefixed IDs without dropping security).
- Skip smooth scroll on modified clicks (ctrl/cmd/shift/alt) so
  browser default behavior (e.g., open in new tab) is preserved.
- Respect prefers-reduced-motion for all smooth scroll navigation
  (hash links in MarkdownViewer and ToC heading clicks in App).

Limit rehypeStripClobberPrefix to remark-gfm footnote IDs (fn-*, fnref-*,
footnote-label) to avoid collisions with raw HTML user-content-* IDs.

feat: add smooth scroll for footnote references

Release for v0.23.2