fix: harden merge UI and improve error handling

- MergeAccounts: guard mutationFn against null token/userId
- MergeAccounts: call checkAdminStatus after successful merge
- MergeAccounts: add fallback when success but tokens null
- MergeAccounts: fix ErrorState showing "expired" for all errors
- MergeAccounts: fix formatCountdown dead ternary and hardcoded English
- MergeAccounts: add staleTime/refetchOnWindowFocus to preview query
- MergeAccounts: remove manual useCallback (React Compiler handles it)
- LinkOAuthCallback: add toast feedback on all error/success paths
- ConnectedAccounts: use invalidateQueries instead of refetch
- ConnectedAccounts: add error state rendering
- ConnectedAccounts/LinkOAuthCallback: share sessionStorage key constants
- i18n: add missing profile.accounts.linking key to zh and fa locales

src/locales/fa.json

@@ -3049,6 +3049,7 @@
       "linkError": "اتصال حساب ناموفق بود",
       "unlinkError": "قطع اتصال ناموفق بود",
       "goToAccounts": "حساب‌های متصل",
+      "linking": "در حال اتصال حساب...",
       "providers": {
         "telegram": "تلگرام",
         "email": "ایمیل",