Add IsBackdropAlwaysActive property to control backdrop behavior on focus loss

[Copilot]

Windows 11 automatically deactivates window backdrop effects (Mica/Acrylic) when a window loses focus, turning them gray. This adds a property to override that behavior.

Changes

• New property: WindowOption.IsBackdropAlwaysActive (default: false)

  • Uses DWMWA_SYSTEMBACKDROP_ALWAYS_ACTIVE attribute (Windows 11 22621+)
  • Follows existing property patterns for event handler management

• Test windows: Added BackdropAlwaysActiveTestWindow and BackdropNormalTestWindow to demonstrate the behavior difference

Usage

<Window xmlns:ws="https://schemas.elecho.dev/wpfsuite"
        ws:WindowOption.Backdrop="Acrylic"
        ws:WindowOption.IsBackdropAlwaysActive="True"
        Background="Transparent">
    <!-- Backdrop remains active when window loses focus -->
</Window>

Original prompt

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This section details on the original issue you should resolve

<issue_title>当窗体失去焦点时，窗体的BackDrop不起作用。</issue_title>
<issue_description>当设置窗体的BakcDrop属性后，窗体变得很漂亮。但当窗体失去焦点时，窗体的背景就便会简单的“灰白”色了，不再有Acrylic等效果。
请问该如何解决这个问题？</issue_description>

<agent_instructions>新增一个属性让用户可以用于控制是否失去焦点时backdrop不生效</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes 当窗体失去焦点时，窗体的BackDrop不起作用。 #31

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