feat: in-app dark mode toggle (System / Light / Dark)

- AppearanceManager with UserDefaults persistence
- Three modes: System (follows iOS), Light, Dark
- Toggle in Home screen profile menu under "Appearance"
- Applied via .preferredColorScheme at app root
- Persists across app launches

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/PlatformApp.swift

@@ -3,11 +3,14 @@ import SwiftUI
 @main
 struct PlatformApp: App {
     @State private var authManager = AuthManager()
+    @State private var appearance = AppearanceManager()
 
     var body: some Scene {
         WindowGroup {
             ContentView()
                 .environment(authManager)
+                .environment(appearance)
+                .preferredColorScheme(appearance.mode.colorScheme)
         }
     }
 }