Add recurring tasks (daily/weekly/monthly repeat)

[MeisnerDan]

Extend the task schema to support recurrence patterns. Users should be able to set a task to repeat daily, weekly, monthly, or on a custom schedule. When a recurring task is marked done, it automatically generates the next instance. This is essential for habit tracking and routine work management.

Acceptance Criteria

• Task schema includes recurrence field with options: none, daily, weekly, monthly, custom
• For custom recurrence, support interval + day-of-week configuration
• When a recurring task is marked done, a new task is automatically created for the next period
• Recurring tasks show a "repeats" badge on task cards
• Task detail page shows next recurrence date
• Users can "break the chain" (mark as done without creating next instance)
• Completed recurring tasks are linked to their parent in the data structure
• Unit tests for recurrence calculation logic (e.g., weekly on Monday)
• Activity log tracks recurring task instance generation