[S4.1-10] Mobile and Accessibility Baseline Pass

[humanauction]

Labels: stage-4, frontend, a11y, mobile
Depends on: S4.1-09

Acceptance Criteria:

• No horizontal overflow at small mobile widths.
• Touch targets are comfortably tappable.
• Keyboard navigation works on primary flows.
• Basic ARIA/labeling and contrast issues are resolved.