feat: 新增维护模式 + 修复注册关闭页面标题显示问题

- 修复注册关闭时标题“创建账号”和副标题仍显示的问题，三态互斥（维护中 > 关闭注册 > 正常）
- 新增 maintenance.enabled 站点设置，默认关闭，仅 Owner 可见可调节
- 维护中间件：全局拦截，维护期间仅站长可访问，白名单放行登录相关路径
- 登录服务：维护期间仅 Owner 角色可登录，Register 优先检查维护状态
- Nav 模板新增维护通知条（黄色横幅）
- 管理后台站点设置新增“启用维护模式”开关
- BuildPageData 自动注入 IsMaintenance 到所有页面模板

internal/router/deps_core.go

@@ -14,6 +14,7 @@ import (
 type dependencies struct {
 	authCtrl        *controller.AuthController
 	authMdw         *middleware.AuthMiddleware
+	maintenanceMdw  *middleware.MaintenanceMiddleware
 	settingsCtrl    *controller.SettingsController
 	messageCtrl     *controller.MessageController
 	postCtrl        *controller.PostController

internal/router/deps_extra.go

@@ -4,6 +4,7 @@ import (
 	"metazone.cc/metalab/internal/config"
 	"metazone.cc/metalab/internal/controller"
 	adminCtrl "metazone.cc/metalab/internal/controller/admin"
+	"metazone.cc/metalab/internal/middleware"
 	"metazone.cc/metalab/internal/repository"
 	"metazone.cc/metalab/internal/service"
 
@@ -41,8 +42,11 @@ func buildDeps(db *gorm.DB, cfg *config.Config, siteSettings *config.SiteSetting
 	spaceService := service.NewSpaceService(userRepo, postRepo)
 	spaceCtrl := controller.NewSpaceController(spaceService)
 
+	maintenanceMdw := middleware.NewMaintenanceMiddleware(cfg, siteSettings)
+
 	return &dependencies{
-		authCtrl: authCtrl, authMdw: authMdw, settingsCtrl: settingsCtrl,
+		authCtrl: authCtrl, authMdw: authMdw, maintenanceMdw: maintenanceMdw,
+		settingsCtrl: settingsCtrl,
 		messageCtrl: messageController, postCtrl: postCtrl, spaceCtrl: spaceCtrl,
 		studioCtrl: studioCtrl,
 		adminPostCtrl: adminPostCtrl,

internal/router/router.go

@@ -15,6 +15,9 @@ func Setup(r *gin.Engine, db *gorm.DB, cfg *config.Config, siteSettings *config.
 
 	deps := buildDeps(db, cfg, siteSettings)
 
+	// 维护模式中间件（全局，在路由匹配之前）
+	r.Use(deps.maintenanceMdw.Handler())
+
 	setupFrontendRoutes(r, cfg, deps)
 	setupAPIRoutes(r, cfg, deps)
 	setupAdminRoutes(r, cfg, deps)