feat(ui): persist retained window layouts

src/AcDream.App/UI/IRetainedWindowStateController.cs

@@ -0,0 +1,18 @@
+namespace AcDream.App.UI;
+
+/// <summary>Panel state that is not completely described by outer-frame bounds.</summary>
+public readonly record struct RetainedWindowState(
+    bool Collapsed = false,
+    bool Maximized = false,
+    float? PersistedTop = null,
+    float? PersistedHeight = null);
+
+/// <summary>
+/// Optional state seam used by retained-window persistence. Bounds are restored
+/// first; the controller then reapplies collapsed/maximized presentation.
+/// </summary>
+public interface IRetainedWindowStateController
+{
+    RetainedWindowState CaptureWindowState();
+    void RestoreWindowState(RetainedWindowState state);
+}