CLEANUP: Replace deprecated withOpacity() with withValues() in calendar_screen.dart

[Satyanarayana001]

flutter analyze reports deprecated withOpacity() call in
calendar_screen.dart that should be replaced with
.withValues() to avoid precision loss.

Fix: Replace .withOpacity(x) with .withValues(alpha: x)

[Satyanarayana001]

I'd like to work on this issue. I'll replace the deprecated
withOpacity() call with withValues() in calendar_screen.dart
and submit a PR soon!