Add nextDueDate field to TaskResponse for recurring task support

- Add nextDueDate field to TaskResponse model (from API's next_due_date)
- Add effectiveDueDate computed property (nextDueDate ?? dueDate)
- Update DynamicTaskCard, TaskCard to display effectiveDueDate
- Update WidgetDataManager to save effectiveDueDate to widget cache
- Update TaskFormView to use effectiveDueDate when editing
- Fix preview mock data to include nextDueDate parameter

This ensures recurring tasks show the correct next due date after completion
instead of the original due date.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/services/task_service.go

@@ -628,11 +628,22 @@ func (s *TaskService) QuickComplete(taskID uint, userID uint) error {
 	// Update next_due_date and in_progress based on frequency
 	if task.Frequency == nil || task.Frequency.Days == nil || *task.Frequency.Days == 0 {
 		// One-time task - clear next_due_date (completion is determined by NextDueDate == nil + has completions)
+		log.Info().
+			Uint("task_id", task.ID).
+			Bool("frequency_nil", task.Frequency == nil).
+			Msg("QuickComplete: One-time task, clearing next_due_date")
 		task.NextDueDate = nil
 		task.InProgress = false
 	} else {
 		// Recurring task - calculate next due date from completion date + frequency
 		nextDue := completedAt.AddDate(0, 0, *task.Frequency.Days)
+		log.Info().
+			Uint("task_id", task.ID).
+			Str("frequency_name", task.Frequency.Name).
+			Int("frequency_days", *task.Frequency.Days).
+			Time("completed_at", completedAt).
+			Time("next_due_date", nextDue).
+			Msg("QuickComplete: Recurring task, setting next_due_date")
 		task.NextDueDate = &nextDue
 
 		// Reset in_progress to false
@@ -640,7 +651,9 @@ func (s *TaskService) QuickComplete(taskID uint, userID uint) error {
 	}
 	if err := s.taskRepo.Update(task); err != nil {
 		log.Error().Err(err).Uint("task_id", task.ID).Msg("Failed to update task after quick completion")
+		return err // Return error so caller knows the update failed
 	}
+	log.Info().Uint("task_id", task.ID).Msg("QuickComplete: Task updated successfully")
 
 	// Send notification (fire and forget)
 	go s.sendTaskCompletedNotification(task, completion)