fix: remove delete confirmation dialog from thread view (#402)

## Summary

- Removes the `AlertDialog` popup that appeared when tapping delete in thread view
- Deletion now happens immediately, matching the behaviour of the single mail view
- The existing `UndoShell` widget already listens for new `UndoAction` pushes and shows a snack bar with an **Undo** button — no extra UI code needed

Closes #398

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/402

lib/ui/screens/thread_detail_screen.dart

@@ -297,25 +297,6 @@ class _EmailMessageCardState extends ConsumerState<_EmailMessageCard> {
   }
 
   Future<void> _delete() async {
-    final confirmed = await showDialog<bool>(
-      context: context,
-      builder: (ctx) => AlertDialog(
-        title: const Text('Delete email'),
-        content: const Text('Move this email to Trash?'),
-        actions: [
-          TextButton(
-            onPressed: () => Navigator.pop(ctx, false),
-            child: const Text('Cancel'),
-          ),
-          TextButton(
-            onPressed: () => Navigator.pop(ctx, true),
-            child: const Text('Delete'),
-          ),
-        ],
-      ),
-    );
-    if (!mounted) return;
-    if (confirmed == true) {
     final repo = ref.read(emailRepositoryProvider);
     // Fetch data first for IMAP undo support
     final original = await repo.getEmail(widget.email.id);
@@ -340,4 +321,3 @@ class _EmailMessageCardState extends ConsumerState<_EmailMessageCard> {
     }
   }
 }
-}