Add background task for overnight widget data refresh

- Create BackgroundTaskManager that schedules refresh at random time
  between 12:00 AM - 4:00 AM local time
- Fetches tasks from API, writes to DataManager and shared App Group
- Reloads widget timelines after refresh
- Register task in AppDelegate, schedule when app goes to background
- Add BGTaskSchedulerPermittedIdentifiers and fetch/processing modes to Info.plist

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

iosApp/iosApp/PushNotifications/AppDelegate.swift

@@ -1,5 +1,6 @@
 import UIKit
 import UserNotifications
+import BackgroundTasks
 import ComposeApp
 
 class AppDelegate: NSObject, UIApplicationDelegate, UNUserNotificationCenterDelegate {
@@ -14,6 +15,9 @@ class AppDelegate: NSObject, UIApplicationDelegate, UNUserNotificationCenterDele
         // Register notification categories for actionable notifications
         NotificationCategories.registerCategories()
 
+        // Register background tasks for overnight data refresh
+        BackgroundTaskManager.shared.registerBackgroundTasks()
+
         // Request notification permission
         Task { @MainActor in
             await PushNotificationManager.shared.requestNotificationPermission()