fix: guard threadEmails.last against empty list

Add an isEmpty check before accessing threadEmails.last to prevent
StateError when emails are deleted between query and access.

Closes #341

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

lib/data/repositories/email_repository_impl.dart

@@ -156,6 +156,7 @@ class EmailRepositoryImpl implements EmailRepository {
       return;
     }
 
+    if (threadEmails.isEmpty) return;
     final latest = threadEmails.last;
 
     // Collect unique participants across the whole thread.