feat(ui): the creator's six corrections

1. Stream Info + YPP rails are YouTube surfaces → new `youtube` token #ff0000. It is documented
   as brand-only: live state stays `primary`, so the two reds never mean the same thing.
2. EDIT toggle: the checked trigger set the SAME colour as the resting face, so clicking it
   changed nothing visible. Resting is now the neutral surface with muted text; armed is blue with
   dark text, and the label follows the fill.
3. Active scene outline is orange again (`attention`) — it marks the scene you work on, not a
   selection; blue made it read as "selected". Live stays red.
4. Scene names wear the same pill as the hotkey chips (black, 10px radius, 8×2 padding), so both
   labels stay readable over any thumbnail.
5. REC / ON-AIR pills and the running stop button read red while armed/live — including test
   mode, where the top bar is gold. New `RecordPillToggle` style (its own template, because
   SegmentToggle hard-codes a transparent face); the stop button's grey `#333` literal is gone.
6. The properties list had `MaxHeight="420"` inside an Auto row, so it scrolled while the window
   had space left. The row is `*` now and the cap is gone.

Guard: UiShellTests pins all six to the markup and names the one that regresses.

Controls/LeftPanel.xaml

@@ -49,7 +49,7 @@
                             <RowDefinition Height="Auto"/>
                             <RowDefinition Height="Auto"/>
                             <RowDefinition Height="Auto"/>
-                            <RowDefinition Height="Auto"/>
+                            <RowDefinition Height="*"/>
                         </Grid.RowDefinitions>
 
                         <!-- LAYERS header + Add button -->
@@ -300,7 +300,7 @@
                     <TextBlock Grid.Row="6" Text="Select an item to edit its properties" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="12"
                                Margin="8,4,0,0" TextWrapping="Wrap"
                                Visibility="{Binding SelectedElement, Converter={StaticResource NullToVis}}"/>
-                    <ScrollViewer x:Name="ElementsScrollViewer" Grid.Row="6" MaxHeight="420" VerticalScrollBarVisibility="Auto" Background="Transparent"
+                    <ScrollViewer x:Name="ElementsScrollViewer" Grid.Row="6" VerticalScrollBarVisibility="Auto" Background="Transparent"
                                   Visibility="{Binding SelectedElement, Converter={StaticResource NotNullToVis}}">
                         <StackPanel x:Name="ElementsPanel" Margin="8,4,0,0">
                             <!-- Opacity + mirror/shape (draggable elements: image, webcam) -->