design: the last VM-side colours come from tokens (ThemeBrush)

The state colours the view-models hand to bindings — top bar, REC/ON-AIR dots, health banner,
meter bands, toast tints — were hex literals in C#. They now resolve from Themes/Tokens.xaml
through `Helpers/ThemeBrush`, so the values live in one place. Without an Application (unit tests)
the helper returns a neutral grey: visibly wrong rather than silently plausible.

Four tokens the code needed and the document lacked, added to DESIGN.md (so the doc-is-the-contract
guard demanded their brushes): info-container #3a3b52, success-container #1d5c38,
warning-container #b8860b, test-container #9c6f1c (the test-stream gold bar).

Also fixed by this: the top bar still painted the RETIRED brand red #e94560 when live — the
standard's #ff5451 now, and `OnAirSignTests` (which pinned the old value) says so. BarOn/BarOff
became properties rather than static fields, because a static field would resolve tokens before
the Application exists.

Verified: DesignTokensTests, GameMeterHonestyTests, NotificationAreaIntegrationTests,
OnAirSignTests, SessionTeardownTests — 38 tests green.

ViewModels/MainViewModel.Streaming.Operations.cs

@@ -316,7 +316,9 @@ public partial class MainViewModel : ViewModelBase
             return;
         }
 
-        HealthIssueBackground = report.IsError ? "#93000a" : "#b8860b";
+        HealthIssueBackground = report.IsError
+            ? ThemeBrush.Hex("Brush.ErrorContainer")
+            : ThemeBrush.Hex("Brush.WarningContainer");
         HealthIssueBanner = report.Text;
     }
 
@@ -486,8 +488,12 @@ public partial class MainViewModel : ViewModelBase
         var live = IsLive;
         // A live TEST broadcast wears gold, not the live red — the TEST badge +
         // gold are the "this is a rehearsal" cue (TASK 41).
-        TopBarBackground = live ? IsTestStream ? "#9c6f1c" : "#ff5451" : "#16213e";
-        PreviewGlowBrush = live ? IsTestStream ? "#9c6f1c" : "#ff5451" : "Transparent";
+        TopBarBackground = live
+            ? ThemeBrush.Hex(IsTestStream ? "Brush.TestContainer" : "Brush.Primary")
+            : ThemeBrush.Hex("Brush.SurfaceContainer");
+        PreviewGlowBrush = live
+            ? ThemeBrush.Hex(IsTestStream ? "Brush.TestContainer" : "Brush.Primary")
+            : "Transparent";
         PreviewGlowThickness = live ? new Thickness(3) : new Thickness(0);
 
         if (!live) _healthPollTimer.Stop();