fix(sidebar): show collapsible groups in mobile drawer

[Thereallo1026]

Fixes #366.

Summary

• Treat the open mobile sidebar drawer as an expanded sidebar root
• Add the named group/sidebar ancestor contract to the mobile drawer
• Preserve sidebar data attributes on mobile so collapsible group content can resolve expanded-state styles

pnpm --filter @cloudflare/kumo exec tsc --noEmit for testing. Manually verified at mobile viewport width that collapsible group children render in the sidebar drawer.

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• Reviews
  • bonk has reviewed the change
  • automated review not possible because: external OSS maintainer without bonk access
• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows: verified the Sidebar Collapsible Groups demo at mobile width before and after the fix; ran pnpm --filter @cloudflare/kumo exec tsc --noEmit
  • Additional testing not necessary because: