fix: use variant colour for invert button hover

[puneetdixit200]

Summary

• keep the base button text using the existing on-* contrast token
• add a separate invert-hover text token based on the button variant colour
• apply that token to the invert hover text and border colour
• add a component spec covering the invert hover token behavior

Closes #2.

Validation

• bun run lint
• bun run test (9 files, 48 tests)
• git diff --check

AI assistance

This PR was prepared with AI assistance. I reviewed the component behavior, kept the change scoped to LxButton, added a targeted Vitest spec, and ran the project lint/test checks before submission.