feat(settings): add ChatGPT OAuth device code login for Codex CLI

Add OAuth device code flow for Codex CLI official subscription auth,
allowing users to log in with their ChatGPT account directly from the
agent settings page without using the terminal.

- Backend: two new endpoints (codex_request_device_code, codex_poll_device_code)
  that handle the OpenAI OAuth device code flow and return tokens to frontend
- Frontend: login UI with verification URL, copyable user code, polling status,
  15-minute timeout, and auto-save via existing persistEnv/persistConfig path
- Auth.json written in Codex CLI compatible format (nested tokens, account_id,
  last_refresh) so codex-acp can use OAuth tokens directly
- Show logged-in status and re-login option when tokens are present
- Remove auth.json textarea from Codex settings UI
- i18n: all 10 languages updated with new login-related keys

src/i18n/messages/ko.json

@@ -557,7 +557,21 @@
       "enableFast": "Fast 활성화",
       "enableFastAria": "Codex용 Fast 서비스 계층 활성화",
       "authJsonNative": "auth.json (네이티브)",
-      "configTomlNative": "config.toml (네이티브)"
+      "configTomlNative": "config.toml (네이티브)",
+      "loginButton": "ChatGPT로 로그인",
+      "loginRequesting": "로그인 코드 요청 중...",
+      "loginStep1": "브라우저에서 다음 URL을 열어주세요:",
+      "loginStep2": "아래 코드를 입력하세요:",
+      "loginPolling": "인증 대기 중...",
+      "loginCancel": "취소",
+      "loginSuccess": "로그인 성공, 설정이 저장되었습니다!",
+      "loginFailed": "로그인 실패: {message}",
+      "loginRetry": "재시도",
+      "loginCodeCopied": "코드가 복사되었습니다",
+      "loggedIn": "계정 로그인됨",
+      "loginRelogin": "재로그인 / 계정 전환",
+      "loginTimeout": "로그인 시간 초과, 다시 시도해 주세요",
+      "loginSaveFailed": "로그인 성공했지만 설정 저장 실패"
     },
     "gemini": {
       "authConfig": "Gemini 인증 설정",