- remove the Providers item from sidebar navigation and locale keys
- simplify chat composer by dropping attach/voice action buttons
- support ReactNode titles in credential cards and add provider brand icons
- refine sheet header/footer styling and device-code footer button hierarchy
- disable “Set default” when a model is unconfigured or already default
- add backend `/api/oauth/*` endpoints for provider status, browser/device-code/token login, flow query/polling, and logout
- extend API handler with OAuth flow/state tracking and route registration, plus OAuth unit tests
- implement frontend credentials page/components for OpenAI, Anthropic, and Google Antigravity login/logout
- add OAuth API client and `useCredentialsPage` hook, with new EN/ZH i18n strings
