feat: Add public user information retrieval endpoint

- Introduced a new endpoint to fetch public user information without authentication.
- Implemented UserToPublicUserInfo function to format user data for the response.
- Updated UserService interface and user service implementation to support fetching users by username.
- Enhanced user handler to validate input parameters and check user status before responding.

internal/service/interfaces.go

@@ -19,6 +19,7 @@ type UserService interface {
 	// 用户查询
 	GetByID(ctx context.Context, id int64) (*model.User, error)
 	GetByEmail(ctx context.Context, email string) (*model.User, error)
+	GetByUsername(ctx context.Context, username string) (*model.User, error)
 
 	// 用户更新
 	UpdateInfo(ctx context.Context, user *model.User) error