Fix: remove redundant DateTime month/day args (avoid_redundant_argument_values)

DateTime(2024, 1, 1) → DateTime(2024) since month and day default to 1.

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

test/unit/email_model_test.dart

@@ -83,7 +83,7 @@ void main() {
         accountId: 'acc',
         mailboxPath: 'INBOX',
         uid: 1,
-        receivedAt: DateTime(2024, 1, 1),
+        receivedAt: DateTime(2024),
         from: const [],
         to: const [],
         cc: const [],