fix: snack bar now auto-dismisses after delete in mail detail view (#401)

## Summary

- When deleting a mail from the single Mail View, \`pushAction()\` was called with \`unawaited\` before \`_navigateTo()\`. This meant the UndoShell snack bar fired *after* navigation had already started, showing the snack bar on the destination scaffold mid-transition — which prevented the snack bar's duration timer from starting correctly.
- Fixed by changing \`unawaited(pushAction(...))\` to \`await pushAction(...)\`. Since Riverpod fires \`ref.listen\` synchronously when state changes, the UndoShell now queues the snack bar on the current stable scaffold *before* \`_navigateTo()\` is called. The snack bar then naturally transfers to the destination scaffold and auto-dismisses after 5 seconds as intended.

Closes #399

## Test plan

- [x] All 338 unit/widget tests pass
- [ ] Manually delete a mail from single Mail View and verify the snack bar appears and auto-dismisses after ~5 seconds
- [ ] Verify the Undo button in the snack bar still works

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/401

lib/ui/screens/email_detail_screen.dart

@@ -93,8 +93,7 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
               final destPath = await repo.deleteEmail(widget.emailId);
 
               if (header != null) {
-                unawaited(
-                  ref.read(undoServiceProvider.notifier).pushAction(
+                await ref.read(undoServiceProvider.notifier).pushAction(
                       UndoAction(
                         id: DateTime.now().toIso8601String(),
                         accountId: header.accountId,
@@ -104,7 +103,6 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
                         destinationMailboxPath: destPath,
                         originalEmails: [header],
                       ),
-                      ),
                     );
               }