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/api_client.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"net/http"
 	"net/url"
+	"strconv"
 )
 
 type apiClient struct {
@@ -78,6 +79,12 @@ func (c *apiClient) List(filter ListFilter) ([]*models.Node, error) {
 			q.Add("rel", string(r.Type)+":"+r.Target)
 		}
 	}
+	if filter.HasDueDate {
+		q.Set("has_due_date", "true")
+	}
+	if filter.DueWithin != nil {
+		q.Set("due_within", strconv.Itoa(*filter.DueWithin))
+	}
 	path := "/nodes"
 	if len(q) > 0 {
 		path += "?" + q.Encode()