Simplify Codex takeover notice copy and match hint styling

Restyle the proxy-takeover notice in the Codex editor from the boxed
Alert to the amber inline-hint style used by the endpoint hints, and drop
implementation jargon (127.0.0.1 / PROXY_MANAGED) that users could not
interpret. The notice and the auth/config hints now simply state that the
form shows the stored provider config rather than the proxy-managed live
config. i18n synced across en/zh/ja/zh-TW.