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.go

@@ -56,7 +56,9 @@ type UpdateInput struct {
 // Tag filters (Target == "") match by rel_name prefix.
 // Edge filters (Target != "") are resolved to node IDs.
 type ListFilter struct {
-	Rels []RelInput
+	Rels       []RelInput
+	HasDueDate bool // when true, only return nodes that have a due date set
+	DueWithin  *int // when non-nil, only return nodes due within this many days (includes overdue)
 }
 
 // RelInput is a typed, directed rel with a target that may be a name or node ID.