Pre-Populate Deadline Description When Editing

Issue Description:
Upcoming deadlines include an edit option that can be accessed by clicking the edit icon. Currently, when a user selects this option, the description textarea is empty, which makes the form behave like a new deadline description is being created rather than an existing one being edited.

The edit form should be updated so that the current deadline description is automatically displayed in the textarea when the user begins editing. This allows the user to modify the existing text instead of having to re-enter the entire description.

Expected Behavior:
When the edit icon is clicked, the textarea should be pre-populated with the existing deadline description. The user should be able to make changes and save the updated description.

UI:

Inspect the deadline edit form and the edit-icon flow first, then trace where the existing deadline description is available to that form. Done means the textarea shows the current description on edit, allows changes, and saves the updated text without requiring re-entry.

Written by the indexing model from the issue text.