Add high-contrast mode overrides for TagChip [b#098]

[greatest0fallt1me]

Description

This is a ui/ux issue for the GrantFox FWC26 campaign. Support prefers-contrast: more on TagChip with explicit border+text overrides.

Requirements and Context

• Implement per the description above
• Add focused tests for the change
• Document any API/visible changes
• Adhere to repo's lint and code style
• Must be secure, tested, and documented

Suggested Execution

1. Fork the repo and create a branch
2. Implement changes touching:
   • src/pages/TagChip.tsx
   • src/styles/contrast.css
3. Test and commit; open a PR with Closes #<issue-number> in the body

Guidelines

• Responsive across all breakpoints
• WCAG 2.1 AA accessibility
• Design-token + dark-mode consistency
• Clear documentation and inline comments
• Timeframe: 96 hours