fix: rebuild responsive sidebar navigation - #13

Summary

• add reusable Sidebar navigation sections and items
• replace the docs mobile navigation overlay with the Drawer component
• keep only the active mobile section open by default and close after navigation
• add touch sizing, focus management, responsive browser coverage, and documentation

Validation

• pnpm test
• pnpm typecheck
• pnpm format:check
• pnpm test:a11y