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/serve/server.go

@@ -6,6 +6,7 @@ import (
 	"axolotl/store"
 	"encoding/json"
 	"net/http"
+	"strconv"
 	"strings"
 )
 
@@ -84,6 +85,14 @@ func (s *server) listNodes(w http.ResponseWriter, r *http.Request) {
 	if v := q.Get("mention"); v != "" {
 		filter.Rels = append(filter.Rels, service.RelInput{Type: models.RelMentions, Target: v})
 	}
+	if q.Get("has_due_date") == "true" {
+		filter.HasDueDate = true
+	}
+	if v := q.Get("due_within"); v != "" {
+		if n, err := strconv.Atoi(v); err == nil {
+			filter.DueWithin = &n
+		}
+	}
 	nodes, err := svc.List(filter)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, err.Error())