Instance-wide (global) info banner and maintenance mode (#36571)

The banner allows site operators to communicate important announcements
(e.g., maintenance windows, policy updates, service notices) directly
within the UI.

The maintenance mode only allows admin to access the web UI.

* Fix #2345
* Fix #9618

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/setting/config_option_instance.go

@@ -0,0 +1,58 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"time"
+
+	"code.gitea.io/gitea/modules/setting/config"
+)
+
+// WebBannerType fields are directly used in templates,
+// do remember to update the template if you change the fields
+type WebBannerType struct {
+	DisplayEnabled bool
+	ContentMessage string
+	StartTimeUnix  int64
+	EndTimeUnix    int64
+}
+
+func (b WebBannerType) ShouldDisplay() bool {
+	if !b.DisplayEnabled || b.ContentMessage == "" {
+		return false
+	}
+	now := time.Now().Unix()
+	if b.StartTimeUnix > 0 && now < b.StartTimeUnix {
+		return false
+	}
+	if b.EndTimeUnix > 0 && now > b.EndTimeUnix {
+		return false
+	}
+	return true
+}
+
+type MaintenanceModeType struct {
+	AdminWebAccessOnly bool
+	StartTimeUnix      int64
+	EndTimeUnix        int64
+}
+
+func (m MaintenanceModeType) IsActive() bool {
+	if !m.AdminWebAccessOnly {
+		return false
+	}
+	now := time.Now().Unix()
+	if m.StartTimeUnix > 0 && now < m.StartTimeUnix {
+		return false
+	}
+	if m.EndTimeUnix > 0 && now > m.EndTimeUnix {
+		return false
+	}
+	return true
+}
+
+type InstanceStruct struct {
+	WebBanner       *config.Option[WebBannerType]
+	MaintenanceMode *config.Option[MaintenanceModeType]
+}