fix(ui): guard ref.read with mounted checks in _delete after async gaps

After showDialog and after the two repo awaits (getEmail/deleteEmail),
the widget may have been disposed — calling ref.read on a disposed
ConsumerStatefulElement throws "Cannot use 'ref' after the widget was
disposed." Add if (!mounted) return; at both points.

Fixes #80

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

lib/ui/screens/thread_detail_screen.dart

@@ -261,6 +261,7 @@ class _EmailMessageCardState extends ConsumerState<_EmailMessageCard> {
         ],
       ),
     );
+    if (!mounted) return;
     if (confirmed == true) {
       final repo = ref.read(emailRepositoryProvider);
       // Fetch data first for IMAP undo support
@@ -268,6 +269,7 @@ class _EmailMessageCardState extends ConsumerState<_EmailMessageCard> {
 
       final destPath = await repo.deleteEmail(widget.email.id);
 
+      if (!mounted) return;
       if (original != null) {
         unawaited(
           ref.read(undoServiceProvider.notifier).pushAction(