feat: add test button for AI summary notification in settings

[renjfk]

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.