Implemented Calendar date picker - #114

Related Issue

Fixes #88

Description

Updated the deadline creation form to use a native HTML5 date picker (type="date"), allowing users to select dates visually via a calendar pop-up while allowing users to continue using direct keyboard input. Also cleaned up broken HTML code in the template.

Changes Made

• Updated input type to type="date" for visual calendar selection.
• Removed duplicate <input> tag and redundant id="datepicker" attribute.
• Fixed missing closing </div> tag to restore proper Bootstrap form grid layout.
• Removed unnecessary placeholder="mm/dd/yyyy" attribute.
• Updated backend date parsing in deadlineManagement.py to handle ISO format (%Y-%m-%d).

How to Test

1. Navigate to the Create a Deadline Section.
2. Click the calendar icon to the right of the date input field to trigger the browser's calendar picker.
3. Select a date visually, or type a date directly into the field.
4. Verify the form submits properly and layout aligns cleanly across screen sizes.