refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/user/errs.go

@@ -0,0 +1,15 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package user
+
+const (
+	errInvalidParams = "无效的请求参数"
+	errUserNotFound  = "用户不存在"
+	//nolint:gosec // error message, not hardcoded credentials
+	errPasswordMismatch = "用户名或密码错误"
+	//nolint:gosec // error message, not hardcoded credentials
+	errOldPasswordIncorrect = "原密码不正确"
+	//nolint:gosec // error message, not hardcoded credentials
+	errTokenNotFound = "访问令牌不存在"
+)