This pull request adds support for restricting selectable dates in all themed date and date-time picker components by introducing new firstDay and lastDay parameters. These parameters allow developers to specify a date range, preventing users from selecting dates outside of the defined boundaries. The changes are applied consistently across all relevant widgets, and the ThemedCalendar widget is updated to enforce these constraints.

Date range restriction support:

• Added firstDay and lastDay parameters to ThemedCalendar, with logic to disable selection of days outside the specified range. [1] [2]
• Propagated firstDay and lastDay parameters to all picker widgets: ThemedDateRangePicker, ThemedDatePicker, ThemedDateTimeRangePicker, ThemedDateTimePicker, and ThemedDateTimeSteppedPicker, including their constructors and internal calendar usage. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Documentation and versioning:

• Updated the CHANGELOG.md to document the new parameters and their support across all picker widgets.
• Bumped the package version to 7.5.25 in pubspec.yaml.

Example usage:

• Demonstrated usage of the new parameters in the example app for ThemedDateTimeSteppedPicker.