build(winui3): keep SettingsDrawer host deferred + narrow the suspect

Tried re-hosting SettingsDrawer with `Visibility="Collapsed"` (no
RenderTransform / Storyboard this time). Still crashes the XAML parser
at startup with the same HR 0x802b000a.

Narrows the suspect: the crash is inside SettingsDrawer.xaml's
InitializeComponent, not in MainWindow.xaml's hosting of it. Most
likely cause: `IsSelected="True"` on the first NavigationViewItem
fires `OnTabSelectionChanged` during the XAML parse, BEFORE the
SettingsDrawer code-behind has finished construction — the handler
then calls into TabContent which isn't ready, throwing in the parser
context.

Two fixes to try next session:

1. Drop `IsSelected="True"` from XAML and set it programmatically in
   the SettingsDrawer constructor AFTER InitializeComponent returns.
2. Verify the OnTabSelectionChanged signature for WinUI 3 1.8 —
   NavigationView's SelectionChanged is
   `TypedEventHandler<NavigationView, NavigationViewSelectionChangedEventArgs>`
   in 1.8 (might be different from the 1.6 SDK signature I wrote
   against).

For now, the MainWindow's OnSettingsClick is a no-op stub. The drawer
XAML is untouched and ready to re-host once one of the above is
applied.

This commit unblocks the running redesign: dotnet build + run produces
the 1280x780 redesigned shell with proper theming, no crash on
launch.

src/TeamsISO.App.WinUI/Views/MainWindow.xaml

@@ -377,11 +377,14 @@
                 </StackPanel>
             </Border>
 
-            <!-- Settings drawer host removed temporarily to isolate the
-                 XAML parse error blocking launch on WindowsAppSDK 1.8.
-                 The drawer XAML itself is unchanged and remains in
-                 Views/SettingsDrawer.xaml. Re-host once the parse error
-                 is triaged. -->
+            <!-- SettingsDrawer host deferred: even with Visibility=Collapsed
+                 the SettingsDrawer.xaml parse fires at constructor time and
+                 crashes the WinUI 3 XAML parser. Suspect the NavigationView's
+                 IsSelected="True" attribute on its first NavigationViewItem
+                 firing OnTabSelectionChanged before the code-behind is ready.
+                 Re-host after replacing the NavigationView with a simpler
+                 tab strip or after fixing the selection-change handler
+                 signature for WinUI 3 1.8. -->
 
             <!-- ─── Status bar ─── -->
             <Grid Grid.Row="4"

src/TeamsISO.App.WinUI/Views/MainWindow.xaml.cs

@@ -61,13 +61,13 @@ public sealed partial class MainWindow : Window
     }
 
     /// <summary>
-    /// Drawer toggle stubbed pending XAML-parse-error triage. The
-    /// SettingsDrawer XAML itself builds clean and is hosted inline once
-    /// the launch crash is resolved (see Phase 3 of the migration plan).
+    /// Settings drawer toggle. Currently a no-op because the drawer host
+    /// can't be inlined in MainWindow.xaml without crashing the XAML parser;
+    /// see the comment in MainWindow.xaml at the drawer placeholder.
     /// </summary>
     private void OnSettingsClick(object sender, RoutedEventArgs e)
     {
-        // No-op stub until SettingsDrawerHost re-hosts in MainWindow.xaml.
+        // No-op until SettingsDrawer.xaml is simplified for WinUI 3 1.8.
     }
 
     /// <summary>