feat: replace mail protocol with smolmail and rebuild the UI

lib/smol/errors.dart

@@ -0,0 +1,10 @@
+/// Raised for protocol-level failures (malformed envelopes, bad statuses,
+/// unpinned servers); programmer errors keep throwing normally.
+class SmolError implements Exception {
+  final String message;
+
+  const SmolError(this.message);
+
+  @override
+  String toString() => message;
+}