Add Timeout to Deadline Creation Success Message

Issue Description:
When a deadline is successfully created, a flash message is displayed to confirm that the action was completed. Currently, this message remains visible indefinitely and does not disappear unless the page is refreshed or the user navigates away.

Update the flash message behavior so that it automatically disappears after a short period of time.

Expected Behavior:
After a deadline is successfully created, the confirmation message should appear and then automatically fade out or disappear after a few seconds.

UI:

Start at the deadline-creation flow and inspect how its success flash message is rendered and styled; the issue names no file or test. Reproduce a successful creation, then verify the message fades or disappears after a few seconds and remains usable before it does.

Written by the indexing model from the issue text.