feat(ui): assemble MainWindow from the components layer

Top-level chat window composes HonorificHeader, Sidebar, MessageList,
InputBar and StatusBar in the layout from the master spec: header row,
horizontal body (sidebar + main area with messages + input), status
strip pinned to the bottom. Component types are fully qualified through
the Ui.Components prefix so the v1.5.6 Ui.StatusBar type cannot shadow
the new layer through parent-namespace resolution before it is removed.
Toggle is a new-shadow on Window.Toggle so the open path also writes
Config.MainWindowOpen; OnClose covers the close path through the base
behaviour. InputBar.Height is now public so the layout math can reach
it from outside the components folder.

HellionChat/Ui/Components/InputBar.cs

@@ -16,7 +16,7 @@ namespace HellionChat.Ui.Components;
 // picker overlay.
 internal sealed class InputBar
 {
-    private const float Height = 32f;
+    public const float Height = 32f;
     private const float PillHeight = 22f;
     private const float PillPaddingX = 8f;
     private const int BufferCapacity = 500;