feat(ui): apply animated background to all screens via themedBackground modifier

Extract AnimatedSportsBackground components to shared file and update
ThemedBackground modifier to conditionally show animations when enabled
in settings. All views using .themedBackground() now get animated background.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Core/Theme/ViewModifiers.swift

@@ -183,7 +183,15 @@ struct ThemedBackground: ViewModifier {
 
     func body(content: Content) -> some View {
         content
-            .background(Theme.backgroundGradient(colorScheme))
+            .background {
+                if DesignStyleManager.shared.animationsEnabled {
+                    AnimatedSportsBackground()
+                        .ignoresSafeArea()
+                } else {
+                    Theme.backgroundGradient(colorScheme)
+                        .ignoresSafeArea()
+                }
+            }
     }
 }