Adopts ISO week date calculation (Monday-based) for consistency
and accuracy across calendar components. This change ensures
standardized week determination, resolving potential discrepancies
in week-based operations.
Centralizes date calculation logic into a dedicated DateCalculator
class for improved maintainability and testability.
Removes redundant date calculation methods from CalendarManager,
ColumnRenderer, HeaderRenderer and NavigationRenderer, and utilizes
the DateCalculator for these calculations.
Updates components to use DateCalculator for consistent date
handling, including week start determination, date formatting,
and "is today" checks.
