Docs: switch coverage badge to Codecov

[ohmyfelix]

What changed

• switched the README coverage badge link and image from Coveralls to Codecov
• verified the repo already uses the shared nette-tester-coverage-v2 workflow and does not contain a tests/.coveralls.yml file

Why

• completes the remaining Coveralls-to-Codecov cleanup for contributte/tracy
• keeps the repository aligned with Migrate remaining Coveralls usage to Codecov across contributte repos contributte#72 and the pattern used in contributte/vite

Refs contributte/contributte#72

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (master@e2ba756). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #32   +/-   ##
=========================================
  Coverage          ?   51.02%           
=========================================
  Files             ?        5           
  Lines             ?       49           
  Branches          ?        0           
=========================================
  Hits              ?       25           
  Misses            ?       24           
  Partials          ?        0           

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