Avalonia: Fix transparent strip on right/bottom edges of Avalonia window

[Gabriel Dufresne (GabrielDuf)]

This pull request improves the window styling and native integration for the main window on Windows, focusing on more consistent and modern border handling and fixing layout issues when changing window styles. The most important changes are grouped below.

Window Styling and Layout Consistency:

• Updated the DataGridRow style in Styles.Common.axaml to always reserve space for the border with a permanent transparent border, preventing layout shifts when selecting rows. The accent border is now never clipped on the first/last rows.

Native Window Handling Enhancements:

• Added several new Win32 constants and the SetWindowPos P/Invoke to MainWindow.axaml.cs to support more advanced window style and layout changes.
• After updating the window style, now explicitly calls SetWindowPos with SWP_FRAMECHANGED and related flags to trigger a WM_NCCALCSIZE message, ensuring the window's non-client area is recalculated immediately.
• Hooks the WM_NCCALCSIZE message in the window procedure to force the client area to occupy the full window rect, eliminating unwanted glass borders from the WS_THICKFRAME style and ensuring a consistent look.