Fix native Windows maximize for the Avalonia main window

[Gabriel Dufresne (GabrielDuf)]

Summary

• Aero Snap drag-to-top, Win+Up, and DWM maximize animations were broken on the Avalonia main window on Windows — dragging the window to the top of a screen would make it vanish off-screen and force an app restart.
• Root cause is upstream: Avalonia 12 #21160 — WindowDecorations.BorderOnly + ExtendClientAreaToDecorationsHint=true strips WS_CAPTION / WS_THICKFRAME from the window style, which makes DWM disable Aero Snap and all maximize/minimize/restore animations.

Cross-platform safety

All new Windows-only code is gated by OperatingSystem.IsWindows() or only reachable through the WndProc hook that's installed under that guard. NativeMethods P/Invokes are pure declarations that compile on any TFM and are never invoked on macOS / Linux. The macOS and Linux branches of SetupTitleBar are unchanged. The simplified WindowStateProperty observer evaluates identically on non-Windows (the field that was removed was always null there).