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/Features/Settings/ViewModels/SettingsViewModel.swift

@@ -72,6 +72,7 @@ final class SettingsViewModel {
         selectedTheme = .teal
         selectedSports = Set(Sport.supported)
         maxDrivingHoursPerDay = 8
+        AppearanceManager.shared.currentMode = .system
     }
 
     // MARK: - Persistence