fix(connected-accounts): guide the user to enter the existing account's password when merging by email

The backend account-takeover fix now requires the existing account's password
before merging an email that already belongs to another account. Show a clear
message (profile.emailMergePasswordRequired) on that response instead of the
generic "already registered" dead-end, so the legitimate owner knows to enter
that account's password (in both password fields) to complete the merge. i18n
in ru/en/zh/fa.

src/locales/en.json

@@ -4594,7 +4594,8 @@
         "vk": "VK"
       },
       "backToAccounts": "Back to accounts"
-    }
+    },
+    "emailMergePasswordRequired": "This email already belongs to another account. To merge the accounts, enter that account's password."
   },
   "theme": {
     "colors": "Theme Colors",

src/locales/fa.json

@@ -4139,7 +4139,8 @@
         "vk": "VK"
       },
       "backToAccounts": "بازگشت به فهرست حساب‌ها"
-    }
+    },
+    "emailMergePasswordRequired": "این ایمیل قبلاً به حساب دیگری تعلق دارد. برای ادغام حساب‌ها، رمز عبور آن حساب را وارد کنید."
   },
   "theme": {
     "colors": "رنگ‌های تم",

src/locales/ru.json

@@ -5146,7 +5146,8 @@
         "vk": "VK"
       },
       "backToAccounts": "К списку аккаунтов"
-    }
+    },
+    "emailMergePasswordRequired": "Этот email уже привязан к другому аккаунту. Чтобы объединить аккаунты, введите пароль от него."
   },
   "theme": {
     "colors": "Цвета темы",

src/locales/zh.json

@@ -4138,7 +4138,8 @@
         "vk": "VK"
       },
       "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。"
   },
   "theme": {
     "colors": "主题颜色",

src/pages/ConnectedAccounts.tsx

@@ -409,7 +409,12 @@ export default function ConnectedAccounts() {
     },
     onError: (err: { response?: { data?: { detail?: string } } }) => {
       const detail = err.response?.data?.detail;
-      if (detail?.includes('already registered')) {
+      // The email belongs to another account and merging it requires proving
+      // ownership: the backend asks for THAT account's password (account-takeover
+      // fix). Guide the user to enter it rather than showing a dead-end error.
+      if (detail?.includes('merge')) {
+        setEmailError(t('profile.emailMergePasswordRequired'));
+      } else if (detail?.includes('already registered')) {
         setEmailError(t('profile.emailAlreadyRegistered'));
       } else if (detail?.includes('already have a verified email')) {
         setEmailError(t('profile.alreadyHaveEmail'));