feat(ui): persist retained window layouts

src/AcDream.UI.Abstractions/Panels/Settings/UiWindowLayout.cs

@@ -0,0 +1,15 @@
+namespace AcDream.UI.Abstractions.Panels.Settings;
+
+/// <summary>
+/// Complete persisted state for one retained window at one screen resolution.
+/// Position and size are outer-frame pixels; panel flags retain the small amount
+/// of state that geometry alone cannot reproduce reliably.
+/// </summary>
+public readonly record struct UiWindowLayout(
+    float X,
+    float Y,
+    float Width,
+    float Height,
+    bool Visible,
+    bool Collapsed,
+    bool Maximized);