feat: improve date time picker ux for scheduling

[khangkontum]

• Issue: Improve Date & Time Picker UX for Scheduling #3755
• Improve user experience when scheduling with a date time picker instead of a vertical scroll-based selector for day, hour, and minute.

[gemini-code-assist]

Code Review

This pull request refactors the date and time picker functionality within the ActionItemFormSheet by replacing the custom date navigation and CupertinoDatePicker with the calendar_date_picker2 package for date selection and a standard showTimePicker for time. The changes include adding new package dependencies, updating various UI colors to use ResponsiveHelper constants, and enhancing the _formatDueDateWithTime method to conditionally display the year. A review comment points out that DateTime.now() is called multiple times within the CalendarDatePicker2 configuration, recommending it be called once and stored in a variable to ensure consistency, especially when the code runs near midnight.

[beastoin]

lgtm @khangkontum