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/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 {