Fix top-right corner close targets

[aladin7]

Summary

• make the top-right corner a reliable close target for the Windows main window in normal, maximized, and fullscreen states
• apply the same behavior consistently to PicView's Windows dialog windows
• preserve Avalonia close-button hover semantics while handling the outermost corner with DPI-, monitor-, and taskbar-aware Win32 hit testing
• cancel a pending close when the pointer is dragged away, and avoid closing when a drag only ends in the corner

Root cause

PicView uses custom Avalonia window chrome, and the outermost top-right pixel was outside the effective close target in several window states and dialogs. The native system Close command is disabled for this custom chrome, so returning the standard close hit-test value alone did not activate the close action.

Validation

• dotnet build src/PicView.Avalonia.Win32/PicView.Avalonia.Win32.csproj -c Debug -p:Platform=x64 — succeeds with 0 errors
• manually verified exact-corner closing in normal, maximized, fullscreen, and Settings windows
• manually verified dragging from the corner outward does not close
• manually verified dragging from outside into the corner does not close
• git diff --check upstream/dev...HEAD

The repository's full test project is currently blocked by pre-existing stale test mocks and the .NET 11 VSTest/Microsoft Testing Platform mismatch; those files are not changed here.

Fixes #320