feat: open OAuth linking in external browser from Telegram Mini App

- Rewrite OAuthCallback.tsx to handle 3 modes: login, link-browser,
  and link-server (external browser without JWT)
- Add linkServerComplete API method (no JWT, auth via state token)
- Update ConnectedAccounts to use platform.openLink() in Mini App
- Add server-complete endpoint to AUTH_ENDPOINTS (skip Bearer token)
- Enable refetchOnWindowFocus for linked-providers query
- Add returnToTelegram/openTelegram i18n keys (ru, en, zh, fa)

src/locales/en.json

@@ -3616,6 +3616,8 @@
       "goToAccounts": "Connected Accounts",
       "linking": "Linking account...",
       "linkingTelegram": "Connecting Telegram...",
+      "returnToTelegram": "Return to Telegram to continue",
+      "openTelegram": "Open Telegram",
       "providers": {
         "telegram": "Telegram",
         "email": "Email",