Add TestCafe test export format

[raphy78626]

Summary

Add TestCafe as an export target alongside the existing Playwright, Cypress, and Selenium exporters.

Why

TestCafe is popular in enterprises and has its own selector engine. Supporting it broadens Vigil's appeal.

Acceptance Criteria

• New testcafe_exporter.py in backend/testai/export/
• Generates valid TestCafe test files from journey data
• Selector mapping works correctly
• Export option appears in the dashboard
• Basic test coverage for the exporter

Reference

• Existing exporters in backend/testai/export/ for patterns
• TestCafe docs

Difficulty

🟢 Easy — follow existing exporter patterns