Add due dates to tasks #6

Add due dates to tasks, with visual indication for overdue items.

Acceptance criteria

• Tasks can have an optional due date (date only, no time/timezone precision for v1)
• Overdue tasks (past due + not completed) show a visually distinct red badge
• Non-overdue tasks with a due date show it in muted grey text

Backend: #2
Frontend: HillolPias/taskapi-fronend#2