Add a multi-step setup wizard that guides new users through initial
configuration on first run. The wizard covers account credentials,
download client connection (with test), RSS source, media paths,
and optional notification setup.
Backend: new /api/v1/setup/ endpoints with sentinel file mechanism.
Frontend: 7-step wizard with validation and i18n (en/zh-CN).
Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)
Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>
