refactor: extract HealthBanner + promote converters to theme — Phase 2, Commit 13

HealthBanner.xaml takes the Row 1 YouTube-health banner (report-by-exception).

Converters: promoted the 7 value<->visibility converters (BoolToVis, Inverse,
NotNull, Null, Hidden, EnumToBool, AllTrueToVis) from MainWindow.Window.Resources
into Themes/Controls.xaml so extracted UserControls can resolve them. AGENTS rule:
controls cannot reach into a window's own named resources; they resolve from the
app-level merged theme instead. Promoted all of them (not just NotNullToVis) so
every pending extraction keeps working without window-level references.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 2 tracker updated in the same commit.

MainWindow.xaml

@@ -31,21 +31,6 @@
 
     <Window.Resources>
         <ResourceDictionary>
-            <BooleanToVisibilityConverter x:Key="BoolToVis"/>
-        <Helpers:InverseBoolToVisibilityConverter x:Key="InverseBoolToVis"
-                                                  xmlns:Helpers="clr-namespace:ytLive.Helpers"/>
-        <Helpers:NotNullToVisibilityConverter x:Key="NotNullToVis"
-                                              xmlns:Helpers="clr-namespace:ytLive.Helpers"/>
-        <Helpers:NullToVisibilityConverter x:Key="NullToVis"
-                                           xmlns:Helpers="clr-namespace:ytLive.Helpers"/>
-        <Helpers:HiddenBoolToVisibilityConverter x:Key="HiddenBoolToVis"
-                                                 xmlns:Helpers="clr-namespace:ytLive.Helpers"/>
-        <Helpers:EnumToBoolConverter x:Key="EnumToBool"
-                                     xmlns:Helpers="clr-namespace:ytLive.Helpers"/>
-        <Helpers:AllTrueToVisibilityConverter x:Key="AllTrueToVis"
-                                              xmlns:Helpers="clr-namespace:ytLive.Helpers"/>
-
-
 
         <!-- Red dot shown in the taskbar icon while live -->
         <DrawingImage x:Key="LiveOverlay">
@@ -207,14 +192,7 @@
         </Border>
 
         <!-- ═══ MIDDLE: Main Content ═══ -->
-        <!-- YouTube health banner (TASK 5 item 3): report-by-exception — visible
-             ONLY while a warning/error configuration issue exists. Amber for
-             warning, darker red for error (HealthIssueBackground). -->
-        <Border Grid.Row="1" Background="{Binding HealthIssueBackground}"
-                Visibility="{Binding HealthIssueBanner, Converter={StaticResource NotNullToVis}}">
-            <TextBlock Text="{Binding HealthIssueBanner}" Foreground="White" FontSize="13"
-                       FontWeight="SemiBold" Margin="16,8" TextTrimming="CharacterEllipsis"/>
-        </Border>
+        <controls:HealthBanner Grid.Row="1"/>
 
         <Grid Grid.Row="2" Margin="8">
             <Grid.RowDefinitions>