Add Calendar Date Picker for Deadline Creation

Issue Description:
When creating a deadline, users currently have to manually type the date into the input field. The date input should be improved by providing a calendar picker that allows users to select a date visually while still allowing the date to be entered manually.

This would make deadline creation more convenient and reduce the chance of entering a date in an incorrect format.

Expected Behavior:
When the user clicks or focuses on the date field, a calendar should be available for selecting the desired date. Users should still have the option to type the date directly into the field.

UI: