These changes belong with the earlier "preview panel uses /configbuilder/preview"
work (b6e2344) but were dropped from that commit. Folding in now:
- Adds .cb-preview-panel + resize-handle + header/content CSS used by the
renderPreviewSection function shipped in 991631a.
- Removes the obsolete addNavIcon("📄", "JSON Preview", ...) sidebar entry.
- Cleans the cb-sec-preview ID out of the scroll-spy section list.
No behavior change beyond what was already committed — fills in the
visual/CSS gap the committed JS depends on.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
