feat: add Settings page stories

Add stories for all Settings modal sections:
- General (theme toggle)
- ProvidersEmpty (no providers configured)
- ProvidersConfigured (with API keys set)
- ModelsEmpty (no custom models)
- ModelsConfigured (with custom models)

Uses play functions to open the settings modal and navigate to sections.
Added @storybook/test dependency for interaction testing utilities.

Author: ammar-agent