fix: resolve RemoteMD crash and code review issues from PR #411

- Fix dependency version conflict: pin remark-gfm@3.0.1, rehype-raw@6.1.1,
  rehype-sanitize@5.0.1 to versions compatible with react-markdown@8
  (npm was hoisting incompatible v4/v7/v6 from Docusaurus, causing
  'inTable' crash at runtime)
- Fix heading slug generation: add extractText() to recursively extract
  plain text from React children, preventing [object Object] in IDs
- Fix anchor link overlay: replace full-surface position:absolute overlay
  with inline # icon that doesn't block clicks on heading content
- Fix version change anchor continuity: preserve URL hash fragment when
  switching releases so users land at the same section
- Fix stale releasesLoaded state: reset releases state when rawUrl or
  releaseVersions props change to prevent rendering wrong document
- Fix render-time ref mutation: move counter resets into React.useMemo
  to avoid React anti-pattern under concurrent rendering

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: web3jenks