Adds four UIApplicationShortcutItems registered dynamically at launch:
• Add Task — opens the add-task sheet immediately, any tab
• Today — switches to the Today tab
• Calendar — switches to the Calendar tab
• Workout — switches to the Workout tab (only registered if the
workout tab is enabled in settings; re-registers when the setting changes)
Cold-launch and foreground-activation paths both handled via AppDelegate.
ShortcutHandler singleton bridges UIKit callbacks to SwiftUI via @Published.
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