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.

Controls/HealthBanner.xaml

@@ -0,0 +1,12 @@
+<UserControl x:Class="ytLive.Controls.HealthBanner"
+             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+    <!-- 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 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>
+</UserControl>

Controls/HealthBanner.xaml.cs

@@ -0,0 +1,11 @@
+using System.Windows.Controls;
+
+namespace ytLive.Controls;
+
+public partial class HealthBanner : UserControl
+{
+    public HealthBanner()
+    {
+        InitializeComponent();
+    }
+}