WIP: local handoff implementation

Local work on the social.io handoff before merging the claude
worktree branch. Includes the full per-spec Sources/Core/Design
module (8 files), watchOS target under WatchApp/, Live Activity +
widget extension, entitlements, scheme, and asset catalog.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

swift/Sources/App/SocialIOApp.swift

@@ -3,17 +3,36 @@ import SwiftUI
 @main
 struct SocialIOApp: App {
     @State private var model = AppViewModel()
+    @AppStorage("sio.theme") private var themeRawValue = ThemePreference.system.rawValue
 
     var body: some Scene {
-        WindowGroup {
-            MailRootView(model: model)
-                .tint(MailTheme.accent)
-                .onOpenURL { url in
-                    model.apply(url: url)
-                }
-        }
         #if os(macOS)
+        WindowGroup {
+            rootView
+        }
         .defaultSize(width: 1440, height: 900)
+
+        Settings {
+            AppearanceSettingsView()
+                .frame(width: 420, height: 300)
+        }
+        #else
+        WindowGroup {
+            rootView
+        }
         #endif
     }
+
+    private var themePreference: ThemePreference {
+        ThemePreference(rawValue: themeRawValue) ?? .system
+    }
+
+    private var rootView: some View {
+        MailRootView(model: model)
+            .tint(SIO.tint)
+            .preferredColorScheme(themePreference.colorScheme)
+            .onOpenURL { url in
+                model.apply(url: url)
+            }
+    }
 }