feat(windows): add parentWindowHandle parameter to override window handle lookup

[aexei]

Description:

Adds an optional int? parentWindowHandle parameter to pickFiles, pickFile, saveFile, and getDirectoryPath. When provided alongside lockParentWindow: true, the given handle is used directly as hwndOwner instead of calling FindWindowA internally.

This is useful when the app manages multiple windows or the automatic lookup returns the wrong handle.

Usage:

await FilePicker.pickFiles(
  lockParentWindow: true,
  parentWindowHandle: myHwnd, // raw HWND as int
);

If parentWindowHandle is null, behavior is unchanged — _getWindowHandle() is used as before.