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.

Themes/Controls.xaml

@@ -1,5 +1,6 @@
 <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
-                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    xmlns:Helpers="clr-namespace:ytLive.Helpers">
 
     <!-- Base button style -->
     <Style x:Key="YtButton" TargetType="Button">
@@ -631,4 +632,15 @@
         </Setter>
     </Style>
 
+    <!-- Value-to-visibility converters, promoted from MainWindow.Window.Resources
+         so extracted UserControls can resolve them (AGENTS: controls must not
+         reach into a window's own resources; they share the app-level theme). -->
+    <BooleanToVisibilityConverter x:Key="BoolToVis"/>
+    <Helpers:InverseBoolToVisibilityConverter x:Key="InverseBoolToVis"/>
+    <Helpers:NotNullToVisibilityConverter x:Key="NotNullToVis"/>
+    <Helpers:NullToVisibilityConverter x:Key="NullToVis"/>
+    <Helpers:HiddenBoolToVisibilityConverter x:Key="HiddenBoolToVis"/>
+    <Helpers:EnumToBoolConverter x:Key="EnumToBool"/>
+    <Helpers:AllTrueToVisibilityConverter x:Key="AllTrueToVis"/>
+
 </ResourceDictionary>