feat(frontend): add accessibility and i18n foundations

[TFT444]

Summary

Implements the first Silver follow-up across #202 and #203.

• adds English and Spanish message catalogs with English fallback
• adds runtime locale selection, document language updates, and locale-aware date/number helpers
• translates core navigation, page titles, scan controls, status, and theme controls
• adds catalog completeness/fallback tests
• adds skip navigation, semantic dialogs/status regions, inert closed navigation, labeled icon actions, and semantic clickable cards
• adds a static accessibility CI gate and public audit/contributor documentation
• makes ESLint fail on warnings and resolves the existing React hook warnings

Validation

• npm run test:i18n
• npm run test:a11y
• npm run lint (zero warnings)
• npm run build
• website security tests
• git diff --check

Closes #202
Closes #203

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None