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/Core/Theme/Theme.swift

@@ -69,6 +69,65 @@ final class ThemeManager {
     }
 }
 
+// MARK: - Appearance Mode
+
+enum AppearanceMode: String, CaseIterable, Identifiable {
+    case system = "System"
+    case light = "Light"
+    case dark = "Dark"
+
+    var id: String { rawValue }
+
+    var displayName: String { rawValue }
+
+    var iconName: String {
+        switch self {
+        case .system: return "circle.lefthalf.filled"
+        case .light: return "sun.max.fill"
+        case .dark: return "moon.fill"
+        }
+    }
+
+    var description: String {
+        switch self {
+        case .system: return "Match device settings"
+        case .light: return "Always use light mode"
+        case .dark: return "Always use dark mode"
+        }
+    }
+
+    /// Returns the ColorScheme to apply, or nil for system default
+    var colorScheme: ColorScheme? {
+        switch self {
+        case .system: return nil
+        case .light: return .light
+        case .dark: return .dark
+        }
+    }
+}
+
+// MARK: - Appearance Manager
+
+@Observable
+final class AppearanceManager {
+    static let shared = AppearanceManager()
+
+    var currentMode: AppearanceMode {
+        didSet {
+            UserDefaults.standard.set(currentMode.rawValue, forKey: "appearanceMode")
+        }
+    }
+
+    private init() {
+        if let saved = UserDefaults.standard.string(forKey: "appearanceMode"),
+           let mode = AppearanceMode(rawValue: saved) {
+            self.currentMode = mode
+        } else {
+            self.currentMode = .system
+        }
+    }
+}
+
 // MARK: - Theme
 
 enum Theme {