feat: complete login

backend/providers/wechat/response.go

@@ -0,0 +1,16 @@
+package wechat
+
+type ErrorResponse struct {
+	ErrCode int    `json:"errcode,omitempty"`
+	ErrMsg  string `json:"errmsg,omitempty"`
+}
+
+func (r *ErrorResponse) Error() error {
+	return translateError(r.ErrCode, r.ErrMsg)
+}
+
+type AccessTokenResponse struct {
+	ErrorResponse
+	AccessToken string `json:"access_token,omitempty"`
+	ExpiresIn   int    `json:"expires_in,omitempty"` // seconds
+}