test: add Gradio UI smoke test and CI test job

[strbck]

Summary

• Adds tests/test_ui_smoke.py with 5 assertions verifying the Gradio demo Blocks object and key UI components (run_btn, file_input, gallery, log_output) are constructed with the correct Gradio types on startup
• Updates .github/workflows/ci.yml with a new test job that installs lightweight deps (pytest, gradio, numpy) and runs pytest tests/ before the Docker build step
• Heavy ML libraries (insightface, onnxruntime, cv2) are not installed in CI — they're already stubbed by the existing conftest.py

How it works

ui/app.py calls load_models() at module level, so the smoke test uses importlib.util.spec_from_file_location to load the module fresh and patches refacer.models.load_models with a MagicMock before execution. This keeps the test entirely offline and dependency-light.

Reviewer notes

• The test job runs in parallel with the build job; both must pass on PRs targeting main
• No model files or GPU are needed — the test environment only requires gradio to be importable

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