[A11Y-001] Platform: Accessibility audit and WCAG 2.1 AA compliance

## Overview
Conduct a full accessibility audit of the TechMart app and remediate all WCAG 2.1 Level AA violations. Accessibility is a legal requirement in many markets and expands our addressable audience.

## Acceptance Criteria
- [ ] All interactive elements reachable and operable via keyboard alone
- [ ] Focus indicators visible and meet 3:1 contrast ratio
- [ ] All images have descriptive `alt` text (or `alt=""` for decorative images)
- [ ] Color contrast meets 4.5:1 for normal text, 3:1 for large text
- [ ] All form inputs have associated `` elements
- [ ] Modal dialogs: focus trap, Esc to close, return focus on close
- [ ] Announcements for dynamic content (cart updates, toast notifications) via live regions
- [ ] Skip-to-content link visible on focus at top of page
- [ ] No content relies on color alone to convey meaning
- [ ] ARIA roles and attributes are used correctly (validate with axe or similar)
- [ ] Screen reader tested with VoiceOver (macOS/iOS) and NVDA (Windows)

## Tools
- [ ] axe DevTools browser extension scan (zero critical/serious violations)
- [ ] Lighthouse accessibility score ≥ 95

## Depends On
- None — can be worked in parallel with any sprint, but Sprint 5 ensures all features are in place for a complete audit