feat: add test button for AI summary notification in settings #32
Description
Add a test button in AI Settings to test AI summary notification generation, similar to the existing "Test Connection" button.
Details
- Add "Test Summary" button in Summary Notifications section below the prompt field
- Use yesterday's usage data with configured prompt to generate test summary
- Display generated summary in result area with success/failure states and appropriate icons
- Include loading state with progress indicator during generation
- Button disabled when notifications disabled, AI config missing, or prompt empty
- Reuse existing OpenAI service integration and usage aggregation logic from NotificationService
- Follow TestResult pattern for consistent UI display
- Include API error handling and user warnings about potential costs
This allows users to validate their AI summary configuration before enabling scheduled notifications.