feat: account linking and merge UI for cabinet

Add Connected Accounts page (link/unlink OAuth providers), Link OAuth
Callback handler, and Merge Accounts page with subscription comparison
and user choice. Includes API methods, TypeScript types, routes in
App.tsx, navigation from Profile, and i18n for all 4 locales (ru, en,
zh, fa). Merge page works without JWT auth (validated by merge token).

src/locales/en.json

@@ -3598,6 +3598,30 @@
       "promoOffers": "Promo Offers",
       "promoOffersDesc": "Receive special offers and discounts",
       "unavailable": "Notification settings unavailable"
+    },
+    "accounts": {
+      "title": "Connected Accounts",
+      "subtitle": "Manage your sign-in methods",
+      "linked": "Connected",
+      "notLinked": "Not connected",
+      "link": "Connect",
+      "unlink": "Disconnect",
+      "unlinkConfirm": "Are you sure? You won't be able to sign in with this service after disconnecting.",
+      "cannotUnlinkLast": "Cannot disconnect the last sign-in method",
+      "linkSuccess": "Account connected successfully",
+      "unlinkSuccess": "Account disconnected successfully",
+      "linkError": "Failed to connect account",
+      "unlinkError": "Failed to disconnect account",
+      "goToAccounts": "Connected Accounts",
+      "linking": "Linking account...",
+      "providers": {
+        "telegram": "Telegram",
+        "email": "Email",
+        "google": "Google",
+        "yandex": "Yandex",
+        "discord": "Discord",
+        "vk": "VK"
+      }
     }
   },
   "theme": {
@@ -3807,5 +3831,32 @@
       "reason": "Reason",
       "contactSupport": "If you believe this is an error, please contact support."
     }
+  },
+  "merge": {
+    "title": "Merge Accounts",
+    "description": "This sign-in method is already linked to another account. You can merge both accounts into one.",
+    "currentAccount": "Your current account",
+    "foundAccount": "Found account",
+    "authMethods": "Sign-in methods",
+    "subscription": "Subscription",
+    "noSubscription": "No subscription",
+    "traffic": "Traffic",
+    "devices": "Devices",
+    "balance": "Balance",
+    "until": "until {{date}}",
+    "keepThisSubscription": "Keep this subscription",
+    "chooseSubscription": "Choose which subscription to keep",
+    "afterMerge": "After merging",
+    "allAuthMethodsMerged": "All sign-in methods will be combined",
+    "balanceSummed": "Balance: {{amount}} ₽ (combined)",
+    "unselectedSubscriptionDeleted": "The unselected subscription will be deleted",
+    "historyPreserved": "Transaction history will be preserved",
+    "confirm": "Merge Accounts",
+    "cancel": "Cancel",
+    "expired": "The merge link has expired. Please try again.",
+    "success": "Accounts merged successfully!",
+    "error": "Failed to merge accounts. Please try again later.",
+    "expiresIn": "Expires in {{minutes}}",
+    "merging": "Merging..."
   }
 }