Update frontend/CLAUDE.md after the COSS migration: note the sidebar theme
toggle and the always-use-semantic-tokens rule in Theming, add a 'COSS
composition gotchas' section (MenuGroup requirement, DialogPanel padding, no
Card/Avatar size prop, destructive variant, items-start Field, aliases,
ai-elements drift), and dedupe the repeated lucide-react note.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>