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/router.go

@@ -387,3 +387,7 @@ func registerAdminUploadRoutes(adminRouter *gin.RouterGroup) {
 		adminUploadsRouter.GET("/types", upload.GetDistinctUploadTypes)
 	}
 }
+
+var registerFrontend = func(_ *gin.Engine) {
+	// No-op when not embedding frontend
+}