Task 21: Vinkelhus calculation support
Changes:
- Added 'Vinkelhus' checkbox to enable L-shaped building mode
- Added fields for second wing dimensions (wing2Width, wing2Length)
- Visual UI section for second wing with green highlighting
- Real-time total area calculation showing both wings combined
- Backend calculates area for both wings separately and combines:
* Flat roof: wing1_area + wing2_area
* Pitched roof: calculates each wing surface area independently
- Database migration adds is_corner_house, wing2_width, wing2_length columns
- Form labels update dynamically (Vinge 1/Vinge 2) when corner house mode active
- Proper data persistence and loading for corner house configurations
