Move mic to preview row + restructure bottom bar + hide avatar when signed out

Mic button/sound bar/slider moved from footer into the preview pane
(vertically below the Socials+TRAX row). Bottom bar reduced to a single
row: gear icon left, stream stats centered under preview, resolution
dropdown right. Avatar image only loads when IsConnected fires true — no
stale image from a previous session on startup. Build 0w, 247 tests.

MainWindow.xaml.cs

@@ -34,7 +34,8 @@ public partial class MainWindow : Window
         _viewModel.ChatMessages.CollectionChanged += OnChatMessagesChanged;
         UpdateTaskbarOverlay();
         UpdateSelectionOverlay();
-        LoadAvatarImage();
+        AvatarImage.Source = null;
+        AvatarInitial.Visibility = Visibility.Collapsed;
         RestoreWindowGeometry();
         AppLog.Write("MainWindow ctor: end");
     }