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.cs

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