fix(sidebar): wrap ElementHost in WinForms UserControl + cast Explorer.Close event

Two VSTO-only build errors:

1. CustomTaskPanes.Add() requires a System.Windows.Forms.UserControl.
   ElementHost derives from Control, not UserControl. Wrap it inside a
   thin UserControl so the COM adapter is happy.

2. Outlook.Explorer exposes BOTH a Close() method and a Close event with
   the same name; the C# compiler resolves "explorer.Close += handler"
   to the method group. Cast through ExplorerEvents_10_Event to bind
   the event explicitly.

Core/UI build clean. Host project still requires the VSTO build targets
to compile.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/OutlookAddin/Services/SidebarController.cs

@@ -89,13 +89,18 @@ namespace OutlookAddin.Services
             };
             view.DataContext = vm;
 
+            // CustomTaskPanes.Add requires a System.Windows.Forms.UserControl,
+            // so we host the WPF UserControl inside ElementHost which itself
+            // lives inside a thin WinForms UserControl wrapper.
             var element = new ElementHost
             {
                 Child = view,
                 Dock = System.Windows.Forms.DockStyle.Fill
             };
+            var hostControl = new System.Windows.Forms.UserControl();
+            hostControl.Controls.Add(element);
 
-            var pane = _taskPanes.Add(element, PaneTitle, explorer);
+            var pane = _taskPanes.Add(hostControl, PaneTitle, explorer);
             pane.DockPosition = Office.MsoCTPDockPosition.msoCTPDockPositionRight;
             pane.Width = DefaultWidth;
             pane.Visible = true;
@@ -109,9 +114,12 @@ namespace OutlookAddin.Services
             };
             vm.OpenInBrowserRequested += (s, url) => TryOpenInBrowser(url);
 
-            explorer.Close += () => DetachExplorer(key);
+            // Outlook.Explorer exposes BOTH a Close() method and a Close event
+            // on the same name; cast through the events interface to bind the
+            // event handler.
+            ((Outlook.ExplorerEvents_10_Event)explorer).Close += () => DetachExplorer(key);
 
-            _bindings[key] = new ExplorerBinding(pane, element, vm, handler, explorer);
+            _bindings[key] = new ExplorerBinding(pane, hostControl, element, vm, handler, explorer);
 
             // Kick a first lookup based on whatever is selected when the pane opens.
             handler.TriggerNow();
@@ -170,6 +178,7 @@ namespace OutlookAddin.Services
         private sealed class ExplorerBinding : IDisposable
         {
             private readonly CustomTaskPane _pane;
+            private readonly System.Windows.Forms.UserControl _hostControl;
             private readonly ElementHost _element;
             private readonly ExplorerSelectionHandler _handler;
             private readonly Outlook.Explorer _explorer;
@@ -178,12 +187,14 @@ namespace OutlookAddin.Services
 
             public ExplorerBinding(
                 CustomTaskPane pane,
+                System.Windows.Forms.UserControl hostControl,
                 ElementHost element,
                 ReadingPaneViewModel viewModel,
                 ExplorerSelectionHandler handler,
                 Outlook.Explorer explorer)
             {
                 _pane = pane;
+                _hostControl = hostControl;
                 _element = element;
                 ViewModel = viewModel;
                 _handler = handler;
@@ -195,6 +206,7 @@ namespace OutlookAddin.Services
                 try { _handler.Dispose(); } catch { /* ignore */ }
                 try { _pane.Visible = false; } catch { /* ignore */ }
                 try { _element.Dispose(); } catch { /* ignore */ }
+                try { _hostControl.Dispose(); } catch { /* ignore */ }
             }
         }
     }