refactor(plugins): restructure admin and message_gateway into standard layered sub-packages

backend/plugins/drivers/driver_http/frontend_disabled.go

@@ -0,0 +1,12 @@
+//go:build !embed_frontend
+
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package driver_http
+
+import "io/fs"
+
+// frontendAssets returns nil: this binary was built without the embedded frontend bundle,
+// so unmatched routes keep Gin's default 404 and the frontend is served separately.
+func frontendAssets() fs.FS { return nil }