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/MailNotificationWidgetExtension.swift

@@ -0,0 +1,13 @@
+#if os(iOS) && canImport(WidgetKit) && canImport(ActivityKit) && canImport(AppIntents)
+import ActivityKit
+import AppIntents
+import SwiftUI
+import WidgetKit
+
+@main
+struct SocialIOWidgetsExtension: WidgetBundle {
+    var body: some Widget {
+        MailNotificationLiveActivity()
+    }
+}
+#endif