Make sections optional - allow widgets directly on dashboard

[Zalmez]

Currently, all widgets must be contained within sections (Dashboard → Section → Items). This architectural change would make sections optional, allowing widgets to exist directly on the dashboard for more flexible layouts.

Benefits:

• Users can choose section-based or free-form layouts
• More flexibility in dashboard organization
• Simpler dashboards for users who don't need sections

Technical Requirements:

1. Make DashboardItem.SectionId nullable in database schema
2. Update API DTOs and controllers to support section-less items
3. Create non-destructive migration for existing data
4. Update UI rendering logic for both modes
5. Consider a "Container" widget type that acts like current sections