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",