Introduce the LeaderboardSubmissionDialog widget which allows users to submit their team name, GitHub repo URL, and commit SHA for leaderboard consideration.
Features:
- Field validation: Ensure that all required fields are filled before submission.
- SharedPreferences integration: Save the user's last submission values and auto-populate the fields with these values when the dialog is shown next time.
This enhancement provides a smoother user experience by reducing repetitive data entry for subsequent leaderboard submissions.
