Docs: replace Coveralls badge with Codecov

[ohmyfelix]

What changed

• replaced the README coverage badge and link with the Codecov version used in other migrated Contributte packages
• confirmed the repository already uses the shared Codecov-based coverage workflow, so no workflow or config cleanup was needed

Why

• completes the remaining Coveralls-to-Codecov migration for this repository
• keeps contributte/menu-control aligned with Migrate remaining Coveralls usage to Codecov across contributte repos contributte#72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.26%. Comparing base (59a579c) to head (3ad6534).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
- Coverage   99.31%   99.26%   -0.06%     
==========================================
  Files          15       15              
  Lines         293      271      -22     
==========================================
- Hits          291      269      -22     
  Misses          2        2              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.