Improve site UX: dark mode, collapsible TOC, and floating buttons

- Add light/dark theme toggle with localStorage persistence and
  system preference detection (floating button)
- Add back-to-top floating button
- Make sidebar TOC collapsible: level-3+ headings folded by default
- Mobile: TOC becomes a slide-in drawer toggled by a floating button
- Fix GitHub corner icon clipping on mobile scroll
- Use github-actions[bot] identity for CI commits

Co-Authored-By: Claude Code <noreply@anthropic.com>

Author: secnotes

.github/workflows/markdown-to-html.yml

@@ -36,8 +36,8 @@ jobs:
 
     - name: Commit and push changes
       run: |
-        git config --local user.email "action@github.com"
-        git config --local user.name "GitHub Action"
+        git config --local user.email "41898282+github-actions[bot]@users.noreply.github.com"
+        git config --local user.name "github-actions[bot]"
         git add docs/
         if [[ -n $(git status --porcelain docs/) ]]; then
           git commit -m "Auto-update HTML files from Markdown [skip ci]"