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/cmd/list.go

@@ -11,6 +11,8 @@ import (
 
 var lTags, lRels []string
 var lStatus, lPrio, lType, lNamespace, lAssignee, lMention string
+var lDue bool
+var lDueWithin int
 
 var listCmd = &cobra.Command{
 	Use: "list", Short: "List nodes",
@@ -22,6 +24,11 @@ var listCmd = &cobra.Command{
 		}
 
 		var filter service.ListFilter
+		filter.HasDueDate = lDue
+		if lDueWithin >= 0 {
+			n := lDueWithin
+			filter.DueWithin = &n
+		}
 
 		// --tag is an alias for a label filter with no target.
 		for _, tag := range lTags {
@@ -76,4 +83,6 @@ func init() {
 	f.StringVar(&lNamespace, "namespace", "", "filter by namespace")
 	f.StringVar(&lAssignee, "assignee", "", "filter by assignee")
 	f.StringVar(&lMention, "mention", "", "filter by mention")
+	f.BoolVar(&lDue, "due", false, "filter to nodes with a due date")
+	f.IntVar(&lDueWithin, "due-within", -1, "filter to nodes due within N days (includes overdue)")
 }