fix(scoreboard): box follows the rendered strip, opacity proven, plate always settable, dev override

Four reports, three causes:

1. "Background plate has no code behind it" — the plate COLOUR and OPACITY controls were disabled
   until the plate switch was on, which reads as dead controls. They are always settable now; the
   switch is what decides whether a plate is drawn.
2. "Opacity doesn't work" — proven in the renderer test: 20% and 80% produce alphas of ~51 and ~204,
   so the slider reaches the pixels. What was actually wrong was the box.
3. The strip's BOX was fixed at 56px while a wrapped strip renders taller — the frame was scaled
   into the box and looked squashed/clipped. The source's width/height now follow the rendered
   frame, and its Y follows from the edge.
4. "Simulate (dev) should override Show the scoreboard" — it does: the preview gate is
   (_scoreboardEnabled || _scoreboardSimulate), so the dev toggle works even with the product
   setting off.

Not changed: the strip is STATIC by design (all enabled facts always visible, wrap rather than
scroll). If scrolling is wanted after all, that is a presentation mode to add, not a fix.

Controls/LeftPanel.xaml

@@ -502,9 +502,8 @@
                                           IsChecked="{Binding ScoreboardBackgroundEnabled, Mode=TwoWay}"/>
                                 <ComboBox Style="{StaticResource YtComboBox}" Height="24" FontSize="11" Margin="10,0,0,6"
                                           ItemsSource="{Binding ScoreboardColorOptions}" DisplayMemberPath="Name"
-                                          SelectedValuePath="Hex" SelectedValue="{Binding ScoreboardBackgroundColor, Mode=TwoWay}"
-                                          IsEnabled="{Binding ScoreboardBackgroundEnabled}"/>
-                                <DockPanel Margin="10,0,0,6" IsEnabled="{Binding ScoreboardBackgroundEnabled}">
+                                          SelectedValuePath="Hex" SelectedValue="{Binding ScoreboardBackgroundColor, Mode=TwoWay}"/>
+                                <DockPanel Margin="10,0,0,6">
                                     <TextBlock DockPanel.Dock="Left" Text="Opacity" Foreground="{StaticResource Brush.OnSurfaceVariant}"
                                                FontSize="11" Width="60" VerticalAlignment="Center"/>
                                     <TextBlock DockPanel.Dock="Right" Width="42" TextAlignment="Right" FontSize="11"