feat(themes): theme layout record

HellionChat/Themes/ThemeLayout.cs

@@ -0,0 +1,14 @@
+namespace HellionChat.Themes;
+
+// Layout-Werte spiegeln die ImGuiStyleVar-Slots, die HellionStyle pusht.
+public sealed record ThemeLayout(
+    float WindowRounding,
+    float ChildRounding,
+    float PopupRounding,
+    float FrameRounding,
+    float GrabRounding,
+    float TabRounding,
+    float ScrollbarRounding,
+    float WindowBorderSize,
+    float FrameBorderSize
+);