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 {

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

SportsTime/Features/Settings/Views/SettingsView.swift

@@ -17,6 +17,9 @@ struct SettingsView: View {
             // Subscription
             subscriptionSection
 
+            // Appearance Mode (Light/Dark/System)
+            appearanceSection
+
             // Theme Selection
             themeSection
 
@@ -55,6 +58,57 @@ struct SettingsView: View {
         }
     }
 
+    // MARK: - Appearance Section
+
+    private var appearanceSection: some View {
+        Section {
+            ForEach(AppearanceMode.allCases) { mode in
+                Button {
+                    withAnimation(.easeInOut(duration: 0.2)) {
+                        AppearanceManager.shared.currentMode = mode
+                    }
+                } label: {
+                    HStack(spacing: 12) {
+                        // Icon
+                        ZStack {
+                            Circle()
+                                .fill(Theme.warmOrange.opacity(0.15))
+                                .frame(width: 32, height: 32)
+
+                            Image(systemName: mode.iconName)
+                                .font(.system(size: 16))
+                                .foregroundStyle(Theme.warmOrange)
+                        }
+
+                        VStack(alignment: .leading, spacing: 2) {
+                            Text(mode.displayName)
+                                .font(.body)
+                                .foregroundStyle(.primary)
+                            Text(mode.description)
+                                .font(.caption)
+                                .foregroundStyle(.secondary)
+                        }
+
+                        Spacer()
+
+                        if AppearanceManager.shared.currentMode == mode {
+                            Image(systemName: "checkmark.circle.fill")
+                                .foregroundStyle(Theme.warmOrange)
+                                .font(.title3)
+                        }
+                    }
+                    .contentShape(Rectangle())
+                }
+                .buttonStyle(.plain)
+            }
+        } header: {
+            Text("Appearance")
+        } footer: {
+            Text("Choose light, dark, or follow your device settings.")
+        }
+        .listRowBackground(Theme.cardBackground(colorScheme))
+    }
+
     // MARK: - Theme Section
 
     private var themeSection: some View {

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