refactor(router): merge frontend.go and frontend_embedded.go into one file

- Declared package-level `registerFrontend` variable in `router.go` as a no-op fallback
- Configured `frontend.go` under `embed_frontend` build tag to override `registerFrontend` on package initialization
- Removed separate `frontend_embedded.go`
- Resolved related revive linter warnings

internal/router/custom.go

@@ -1,6 +1,7 @@
 // Copyright 2026 Arctel.net
 // SPDX-License-Identifier: Apache-2.0
 
+// Package router 提供 HTTP 路由注册与服务启动
 package router
 
 import (