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/zh-CN.json

@@ -557,7 +557,21 @@
       "enableFast": "启用 Fast",
       "enableFastAria": "Codex 启用 Fast 服务等级",
       "authJsonNative": "auth.json（原生）",
-      "configTomlNative": "config.toml（原生）"
+      "configTomlNative": "config.toml（原生）",
+      "loginButton": "使用 ChatGPT 登录",
+      "loginRequesting": "正在请求登录码...",
+      "loginStep1": "在浏览器中打开以下链接：",
+      "loginStep2": "输入以下代码：",
+      "loginPolling": "等待授权中...",
+      "loginCancel": "取消",
+      "loginSuccess": "登录成功，配置已保存！",
+      "loginFailed": "登录失败：{message}",
+      "loginRetry": "重试",
+      "loginCodeCopied": "已复制代码",
+      "loggedIn": "账号已登录",
+      "loginRelogin": "重新登录 / 切换账号",
+      "loginTimeout": "登录超时，请重试",
+      "loginSaveFailed": "登录成功但配置保存失败"
     },
     "gemini": {
       "authConfig": "Gemini 认证配置",