- Add ModelOnboardingModal component with provider selection, API key input, and model verification flow
- Add API key authentication support to SettingsModal with clear-key functionality
- Trigger onboarding from App.tsx when modelOnboardingComplete setting is false
- Add comprehensive tests for ModelOnboardingModal and SettingsModal authentication
- Document model onboarding flow and API key auth in AGENTS.md and README
- Add CSS styles for onboarding modal, provider cards, and auth section
