refactor: cleanup dead code from reminder system

- Remove permission fallback logic from LocalNotificationService
- Remove unused methods: _scheduleInApp*, _trackFallback, bindInAppReminderHandler
- Remove unused fields: _permissionFallbackTracker, _inAppReminderHandler, _inAppFallbackTimersByEventId, _canDeliverSystemNotification
- Remove unused _showSnoozeOptions from ReminderOverlay
- Remove unused reminderActionExecutor from AuthSessionBootstrapper
- Remove obsolete test files: reminder_permission_fallback_test, reminder_notification_bridge_test, auth_session_bootstrapper_test
- Add native notification grouping (threadIdentifier/groupKey)

apps/lib/features/calendar/reminders/ui/reminder_overlay.dart

@@ -24,7 +24,6 @@ class ReminderOverlay extends StatefulWidget {
 }
 
 class _ReminderOverlayState extends State<ReminderOverlay> {
-  bool _showSnoozeOptions = false;
   OverlayEntry? _overlayEntry;
 
   ReminderPayload? get _currentPayload => widget.queueManager.currentPayload;
@@ -38,9 +37,6 @@ class _ReminderOverlayState extends State<ReminderOverlay> {
   void _hideSnoozeOptions() {
     _overlayEntry?.remove();
     _overlayEntry = null;
-    setState(() {
-      _showSnoozeOptions = false;
-    });
   }
 
   void _showSnoozeDropdown() {
@@ -91,9 +87,6 @@ class _ReminderOverlayState extends State<ReminderOverlay> {
     );
 
     Overlay.of(context).insert(_overlayEntry!);
-    setState(() {
-      _showSnoozeOptions = true;
-    });
   }
 
   void _handleComplete() {