Enable iCloud sync by default, remove user toggle

- CloudKit sync is now always enabled for all users
- Remove useCloudKit setting and toggle from Settings
- Remove CloudKitSyncMonitor usage (package can be removed)
- Remove container switching logic since sync is always on
- Update SharedModelContainer defaults to enable CloudKit

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

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

Shared/Views/DayView/DayViewViewModel.swift

@@ -35,11 +35,6 @@ class DayViewViewModel: ObservableObject {
     init(addMonthStartWeekdayPadding: Bool) {
         self.addMonthStartWeekdayPadding = addMonthStartWeekdayPadding
 
-        DataController.shared.switchContainerListeners.append { [weak self] in
-            guard let self = self else { return }
-            self.getGroupedData(addMonthStartWeekdayPadding: self.addMonthStartWeekdayPadding)
-        }
-
         DataController.shared.addNewDataListener { [weak self] in
             guard let self = self else { return }
             withAnimation {