test: add minimal headless smoke test and run it in CI (#28)

[kenlacroix]

Closes #28 — adds a minimal smoke test and runs it in CI.

What's added

• test/test_smoke.py — exercises the basic, GUI-free logic without opening a window or needing a Bluetooth sensor: utils.sign, the tick ⇄ breathing_rate roundtrip and bounds (config), valid_address(get_sensor_address(MockSensor())), Pacer radius/points within the unit disk, Model buffer construction, and Model.update_ibis_buffer outlier correction (absurd IBI clamped, plausible IBI passes through).
• test/conftest.py — sets QT_QPA_PLATFORM=offscreen before any QApplication is created, plus a session-scoped offscreen QApplication fixture (the Model/Pacer QObjects need a running app, but no real window is shown).
• .github/workflows/test.yml — runs on push/PR (+ manual): installs headless Qt libs, pip install -e .[test], pytest. Mirrors the existing build.yml style.
• pyproject.toml — a test extra (pytest) and pytest config.

Why it's headless-safe

Qt uses the offscreen platform plugin (set in both the workflow env and conftest.py before QApplication), so no display server is needed. The test reuses the in-repo MockSensor, so no Bluetooth. The GUI-launching test/app.py:main() is never collected.

Notes

• The issue referenced openhrv/test_app.py, which doesn't exist; the real tests live under test/, so the smoke test was added there.
• MockSensor is imported as a top-level app module (the test/ dir is on sys.path via conftest.py) rather than from test.app import …, which would collide with the Python standard library's test package and fail collection.

Verified locally: QT_QPA_PLATFORM=offscreen pytest → 6 passed.