feat: 每用户最大连接数改为可配置，默认 30

将 maxConnsPerUser 从硬编码常量(3)改为数据库动态配置项，管理员可在
/admin/vpn 隧道设置中调整，保存后对新连接立即生效。

- model/vpn.go: VpnSetting 新增 MaxConnsPerUser 字段，gorm default:30
- db/db.go: 种子数据设默认 30；旧库回填 0 值为 30
- vpn/tunnel.go: 删除 maxConnsPerUser 常量，改读 VPN.Settings()，兜底 30
- handler/vpn.go: API 响应/请求结构体新增字段，校验范围 1-1000
- VpnView.vue: 隧道设置表单新增"每用户最大连接数"输入框
- zh.ts/en.ts: 新增 maxConnsPerUser 文案，更新首页多设备描述
- docs/client-development.md: 更新常量表为可配置项

internal/db/db.go

@@ -105,21 +105,30 @@ func seedDefaultAdmin(cfg *config.DatabaseConfig) error {
 func seedDefaultVpnSettings() error {
 	var s model.VpnSetting
 	if err := DB.First(&s, model.VpnSettingSingletonID).Error; err == nil {
+		needSave := false
 		if s.Subnet6 == "" {
 			s.Subnet6 = "fd00:dead:beef::/112"
+			needSave = true
+		}
+		if s.MaxConnsPerUser == 0 {
+			s.MaxConnsPerUser = 30
+			needSave = true
+		}
+		if needSave {
 			DB.Save(&s)
 		}
 		return nil
 	}
 	s = model.VpnSetting{
-		ID:               model.VpnSettingSingletonID,
-		Enabled:          false,
-		Subnet:           "192.168.77.0/24",
-		Subnet6:          "fd00:dead:beef::/112",
-		MTU:              1420,
-		InterfaceName:    "",
-		DoLocalIPConfig:  true,
-		DoRemoteIPConfig: true,
+		ID:                model.VpnSettingSingletonID,
+		Enabled:           false,
+		Subnet:            "192.168.77.0/24",
+		Subnet6:           "fd00:dead:beef::/112",
+		MTU:               1420,
+		InterfaceName:     "",
+		DoLocalIPConfig:   true,
+		DoRemoteIPConfig:  true,
+		MaxConnsPerUser:   30,
 	}
 	return DB.Create(&s).Error
 }