🎨 improve offline server detect

model/server.go

@@ -1,6 +1,8 @@
 package model
 
 import (
+	"time"
+
 	pb "github.com/p14yground/nezha/proto"
 )
 
@@ -10,8 +12,9 @@ type Server struct {
 	Name   string
 	Secret string
 
-	Host  *Host
-	State *State
+	Host       *Host
+	State      *State
+	LastActive time.Time
 
 	Stream      pb.NezhaService_HeartbeatServer `gorm:"-" json:"-"`
 	StreamClose chan<- error                    `gorm:"-" json:"-"`