- Add github_username to Settings type (Rust + TS)
- Add device flow commands: github_device_flow_start, github_device_flow_poll, github_get_user
- Replace manual token KeyField with "Login with GitHub" OAuth button
- Show connected state with username after successful OAuth
- Add disconnect functionality to clear OAuth token
- Update mock-tauri.ts with device flow mock handlers
- Update all tests for new Settings shape
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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