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>

iosApp/iosApp/Helpers/WidgetDataManager.swift

@@ -275,7 +275,7 @@ final class WidgetDataManager {
                     description: task.description_,
                     priority: task.priority?.name ?? "",
                     inProgress: task.inProgress,
-                    dueDate: task.dueDate,
+                    dueDate: task.effectiveDueDate, // Use effective date (nextDueDate if set, otherwise dueDate)
                     category: task.category?.name ?? "",
                     residenceName: "", // No longer available in API, residence lookup needed
                     isOverdue: column.name == "overdue_tasks"