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/api/client.ts

@@ -75,6 +75,7 @@ const AUTH_ENDPOINTS = [
   '/cabinet/auth/password/forgot',
   '/cabinet/auth/password/reset',
   '/cabinet/auth/oauth/',
+  '/cabinet/auth/merge/',
 ];
 
 function isAuthEndpoint(url: string | undefined): boolean {