feat(settings): add dark/light/system appearance mode toggle

- Add AppearanceMode enum with system, light, and dark options
- Add AppearanceManager singleton to persist user preference
- Add appearance section in SettingsView with icon and description
- Apply preferredColorScheme at app root for immediate effect
- Include appearance mode in reset to defaults

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

SportsTime/SportsTimeApp.swift

@@ -82,6 +82,7 @@ struct BootstrappedContentView: View {
     @State private var hasCompletedInitialSync = false
     @State private var showOnboardingPaywall = false
     @State private var deepLinkHandler = DeepLinkHandler.shared
+    @State private var appearanceManager = AppearanceManager.shared
 
     private var shouldShowOnboardingPaywall: Bool {
         !UserDefaults.standard.bool(forKey: "hasSeenOnboardingPaywall") && !StoreManager.shared.isPro
@@ -142,6 +143,7 @@ struct BootstrappedContentView: View {
                 break
             }
         }
+        .preferredColorScheme(appearanceManager.currentMode.colorScheme)
     }
 
     @MainActor