- Add repetition_date field to tasks table (migration 018)
- Support pattern-based repetition: day of week, day of month, specific date
- Add 'Через'/'Каждое' mode selector in task form
- Auto-calculate next_show_at from repetition_date on create/complete
- Show calculated next date in postpone dialog for repetition_date tasks
- Update version to 3.3.0
