fix: reset monthSelectedIn when changeMonth is called in custom header

[martijnrusschen]

Fixes #3829

When using monthsShown >= 2 with a custom header, calling changeMonth() would produce inconsistent results depending on which calendar panel the user last selected a date in. This was because the monthSelectedIn state (which tracks the offset for multi-month display) was not being reset when changeMonth was explicitly called.

This fix:

• Adds onMonthSelectedInChange callback prop to Calendar component
• Resets monthSelectedIn to 0 when changeMonth is called, ensuring the target month always appears in the leftmost position
• Adds test to verify the fix

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.22%. Comparing base (bd3ab11) to head (fe8e4b8).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6058      +/-   ##
==========================================
+ Coverage   99.13%   99.22%   +0.08%     
==========================================
  Files          30       30              
  Lines        3586     3590       +4     
  Branches     1535     1519      -16     
==========================================
+ Hits         3555     3562       +7     
+ Misses         29       27       -2     
+ Partials        2        1       -1     

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[TravisJRyan]

Thank you!!