maint: add responsive browser contracts

[karngyan]

Summary

• test documentation layout at phone, tablet, small desktop, and desktop widths
• lock dense desktop rows and readable touch geometry to rendered pixel contracts
• verify 48px coarse pointer hit areas for selection controls
• cover drawer focus, Escape restoration, route navigation, overflow, and compact viewport contrast

Validation

• pnpm --filter @dowel/docs typecheck
• pnpm --filter @dowel/docs test:a11y, 14 passed
• pnpm format:check

Stack

Depends on #14, which depends on #13.