feat: add due date filters for list command

- --due: show only nodes with a due date set
- --due-within N: show only nodes due within N days (includes overdue)

Implemented in service layer with post-fetch filtering, threaded through
API client and server, and exposed via CLI flags.

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

src/service/node_service_impl.go

@@ -234,6 +234,26 @@ func (s *nodeServiceImpl) List(filter ListFilter) ([]*models.Node, error) {
 			result = append(result, n)
 		}
 	}
+
+	if filter.HasDueDate || filter.DueWithin != nil {
+		now := time.Now().UTC().Truncate(24 * time.Hour)
+		filtered := result[:0]
+		for _, n := range result {
+			if n.DueDate == nil {
+				continue
+			}
+			if filter.DueWithin != nil {
+				due := n.DueDate.UTC().Truncate(24 * time.Hour)
+				cutoff := now.AddDate(0, 0, *filter.DueWithin)
+				if due.After(cutoff) {
+					continue
+				}
+			}
+			filtered = append(filtered, n)
+		}
+		result = filtered
+	}
+
 	return result, nil
 }