Update Avalonia for extended window fixes

[wieslawsoltes]

Summary

• update Avalonia from 12.0.0 to 12.0.5
• pick up the Win32 extended-client-area fixes used by Dock's single-tool floating windows
• preserve Dock's intended BorderOnly custom chrome instead of adding a Dock-specific maximize workaround

Avalonia 12.0.4 fixed borders and shadows on Windows 10 in extended mode and corrected min/max sizing to include the window border. Avalonia 12.0.5 additionally fixed window animations in extended mode:

• https://github.com/AvaloniaUI/Avalonia/releases/tag/12.0.4
• Fix borders and shadows on Windows 10 in extended mode AvaloniaUI/Avalonia#21412
• Win32 - Include border size in GetMinMax request AvaloniaUI/Avalonia#21420
• https://github.com/AvaloniaUI/Avalonia/releases/tag/12.0.5
• Win32: Fix window animations in extended mode AvaloniaUI/Avalonia#21498

Validation

• dotnet restore Dock.slnx --nologo
• dotnet build Dock.slnx -c Release --no-restore --nologo --verbosity:minimal
• full Dock.Avalonia headless suite: 613 passed
• full Dock.Avalonia.Themes suite: 63 passed
• focused HostWindow methods/theme-change tests: 8 passed
• focused ThemeManager tests: 23 passed
• DockMvvmSample Release build
• Dock.Avalonia Release builds for net8.0 and net10.0

Fixes #1092