Add next_due_date to admin task views

Display next_due_date field in task list table and task detail page
so admins can see the effective date used for overdue calculations.

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

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

internal/admin/dto/responses.go

@@ -131,6 +131,7 @@ type TaskResponse struct {
 	FrequencyID    *uint    `json:"frequency_id,omitempty"`
 	FrequencyName  *string  `json:"frequency_name,omitempty"`
 	DueDate        *string  `json:"due_date,omitempty"`
+	NextDueDate    *string  `json:"next_due_date,omitempty"`
 	EstimatedCost  *float64 `json:"estimated_cost,omitempty"`
 	ActualCost     *float64 `json:"actual_cost,omitempty"`
 	ContractorID   *uint    `json:"contractor_id,omitempty"`

internal/admin/handlers/task_handler.go

@@ -404,6 +404,10 @@ func (h *AdminTaskHandler) toTaskResponse(task *models.Task) dto.TaskResponse {
 		dueDate := task.DueDate.Format("2006-01-02")
 		response.DueDate = &dueDate
 	}
+	if task.NextDueDate != nil {
+		nextDueDate := task.NextDueDate.Format("2006-01-02")
+		response.NextDueDate = &nextDueDate
+	}
 	if task.EstimatedCost != nil {
 		cost, _ := task.EstimatedCost.Float64()
 		response.EstimatedCost = &cost