Add `frontend-engineer` skill

[niravpatel27]

Summary

Add a frontend-engineer skill that guides building frontend features — covering component design, state management, accessibility, and testing for web UIs.

What this skill should do

• Help design and build UI components
• Guide on state management patterns
• Enforce accessibility best practices (ARIA, keyboard nav, contrast)
• Suggest testing strategies for UI (unit + e2e)
• Review frontend code for common issues

Reference

See existing skills like lfx-ui-builder/SKILL.md for pattern and format.
Each skill lives in its own directory with a SKILL.md file and markdown instructions.

Acceptance Criteria

• frontend-engineer/SKILL.md exists with correct frontmatter
• Skill gives useful output on a sample frontend task