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/Features/Home/HomeView.swift

@@ -3,6 +3,7 @@ import PhotosUI
 
 struct HomeView: View {
     @Environment(AuthManager.self) private var auth
+    @Environment(AppearanceManager.self) private var appearance
     @State private var vm = HomeViewModel()
     @State private var ringAnimated = false
     @Binding var selectedTab: Int
@@ -50,6 +51,18 @@ struct HomeView: View {
                                 }
                             }
                             Divider()
+                            Menu {
+                                ForEach(AppearanceManager.Mode.allCases, id: \.self) { mode in
+                                    Button {
+                                        appearance.mode = mode
+                                    } label: {
+                                        Label(mode.label, systemImage: mode.icon)
+                                    }
+                                }
+                            } label: {
+                                Label("Appearance", systemImage: appearance.mode.icon)
+                            }
+                            Divider()
                             Button(role: .destructive) {
                                 Task { await auth.logout() }
                             } label: {