fix: use dialog context in event delete confirmation

Same navigator mismatch as the chat delete bug — outer context inside
showDialog builder resolves to the wrong navigator ancestor.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/screens/calendar/event_form_sheet.dart

@@ -143,19 +143,19 @@ class _EventFormSheetState extends ConsumerState<EventFormSheet> {
   Future<void> _delete() async {
     final confirmed = await showDialog<bool>(
       context: context,
-      builder: (_) => AlertDialog(
+      builder: (dialogContext) => AlertDialog(
         title: const Text('Delete this event?'),
         actions: [
           TextButton(
-            onPressed: () => Navigator.of(context).pop(false),
+            onPressed: () => Navigator.pop(dialogContext, false),
             child: const Text('Cancel'),
           ),
           TextButton(
-            onPressed: () => Navigator.of(context).pop(true),
+            onPressed: () => Navigator.pop(dialogContext, true),
             child: Text(
               'Delete',
-              style:
-                  TextStyle(color: Theme.of(context).colorScheme.error),
+              style: TextStyle(
+                  color: Theme.of(dialogContext).colorScheme.error),
             ),
           ),
         ],