fix(ui): MainLayout — declare @implements IDisposable so Dispose actually runs (HIGH)

The component had a Dispose() method that detached ThemeState/LocaleState
event handlers, but it was never invoked because the @implements directive
was missing. Each navigation through the layout leaked two subscriptions
on the singleton state objects.

Other state subscribers (NavBody.razor, AnomalyFeed.razor) already declare
the directive correctly.

src/Marathon.UI/MainLayout.razor

@@ -1,4 +1,5 @@
 @inherits LayoutComponentBase
+@implements IDisposable
 @inject ThemeState ThemeState
 @inject LocaleState LocaleState
 @inject IStringLocalizer<SharedResource> L