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/fr.json

@@ -557,7 +557,21 @@
       "enableFast": "Activer Fast",
       "enableFastAria": "Activer le niveau de service Fast pour Codex",
       "authJsonNative": "auth.json (natif)",
-      "configTomlNative": "config.toml (natif)"
+      "configTomlNative": "config.toml (natif)",
+      "loginButton": "Se connecter avec ChatGPT",
+      "loginRequesting": "Demande du code de connexion...",
+      "loginStep1": "Ouvrez l'URL suivante dans votre navigateur :",
+      "loginStep2": "Entrez le code ci-dessous :",
+      "loginPolling": "En attente d'autorisation...",
+      "loginCancel": "Annuler",
+      "loginSuccess": "Connexion réussie, configuration enregistrée !",
+      "loginFailed": "Échec de la connexion : {message}",
+      "loginRetry": "Réessayer",
+      "loginCodeCopied": "Code copié",
+      "loggedIn": "Compte connecté",
+      "loginRelogin": "Reconnecter / Changer de compte",
+      "loginTimeout": "Connexion expirée, veuillez réessayer",
+      "loginSaveFailed": "Connexion réussie mais échec de la sauvegarde de la configuration"
     },
     "gemini": {
       "authConfig": "Configuration d’authentification Gemini",