Add macOS to CI matrix

[mokagio]

Summary

• add macos-latest to the existing CI matrix
• keep the current Python 3.9-3.12 coverage unchanged
• expand issue macOS CI #138 into an implementation PR

Validation

• uv run --extra test pytest tests/test_countdown.py -q
• uv run --extra test pytest -q

Closes #138.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.85%. Comparing base (abba178) to head (19e602e).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #142   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files          43       43           
  Lines        3534     3535    +1     
=======================================
+ Hits         3529     3530    +1     
  Misses          5        5           

Flag	Coverage Δ
unittests	99.85% <100.00%> (+<0.01%)	⬆️

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[robamu]

Thanks :)