perf(calendar): use const fn

[EdJoPaTo]

There are likely many more places where Style::default() → Style::new() allows for const.

Also, do the comparison without as u8. Stays the same at runtime and is cleaner code.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.4%. Comparing base (1126478) to head (b40d227).

Files	Patch %	Lines
src/widgets/calendar.rs	83.3%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1039   +/-   ##
=====================================
  Coverage   89.4%   89.4%           
=====================================
  Files         61      61           
  Lines      15430   15430           
=====================================
  Hits       13799   13799           
  Misses      1631    1631           

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