feat(connected-accounts): email-merge confirmation via emailed one-time code

When linking an email that belongs to another account, the backend now mails a
one-time code to it (merge_verification='email_code') instead of asking for that
account's password. Add a 6-digit code step: on the email_code response show the
code input, verify via authApi.verifyEmailMerge → on success redirect to /merge.
Preview/execute already go through apiClient (JWT), so they satisfy the new
initiator-bound merge endpoints. i18n in ru/en/zh/fa.

src/locales/zh.json

@@ -4139,7 +4139,12 @@
       },
       "backToAccounts": "返回账户列表"
     },
-    "emailMergePasswordRequired": "该邮箱已属于另一个账户。要合并账户，请输入该账户的密码。"
+    "emailMergePasswordRequired": "该邮箱已属于另一个账户。要合并账户，请输入该账户的密码。",
+    "emailMergeCodeSent": "确认码已发送至该邮箱。",
+    "emailMergeCodeDescription": "该邮箱已属于另一个账户。我们已向其发送验证码——请输入以确认所有权并合并账户。",
+    "emailMergeCodeLabel": "邮件中的验证码",
+    "emailMergeConfirm": "确认并合并",
+    "emailMergeCodeInvalid": "验证码无效或已过期"
   },
   "theme": {
     "colors": "主题颜色",