feat(site): add legacy auto sync helper and tritan cleanup - #25180

Summary

• Add legacyAutoToSync to map legacy auto-family theme preferences to light and dark sync pairs.
• Expand colorblind theme tests for legacy auto-family handling.
• Move tritan success and git-added surfaces onto the sky-blue palette in theme roles and CSS variables.

Dependencies

• None. This can merge independently before the dropdown work.

Validation

• pnpm -C site exec vitest run --project=unit src/theme/colorblind.test.ts
• pnpm -C site lint:types
• Pre-commit hook passed on the branch commit.