A11Y-001: Accessibility audit — WCAG 2.1 AA compliance across all pages

## Overview
Conduct an accessibility audit of TechMart and resolve all WCAG 2.1 AA violations. Focus on keyboard navigation, screen reader compatibility, and color contrast.

## Acceptance Criteria
- [ ] All interactive elements reachable and operable via keyboard alone
- [ ] All images have meaningful alt text (or empty alt for decorative)
- [ ] Color contrast ratio ≥ 4.5:1 for all body text
- [ ] Modal dialogs trap focus correctly and can be dismissed with Escape
- [ ] ARIA labels on icon-only buttons (cart, filter buttons)
- [ ] axe or WAVE audit shows zero 'critical' or 'serious' violations

## Notes
No upstream dependencies — can be worked in parallel with any sprint.

Start by running an axe or WAVE audit across all TechMart pages, then review the reported keyboard, screen-reader, contrast, image, modal, and icon-button issues. Done means every listed acceptance criterion is met and the audit reports zero critical or serious violations.

Written by the indexing model from the issue text.