feat: replace mail protocol with smolmail and rebuild the UI

lib/shared/utils/snackbar.dart

@@ -0,0 +1,15 @@
+import "package:flutter/material.dart";
+
+/// Errors get the error color explicitly — the theme's default SnackBar is
+/// neutral, so a plain SnackBar(content: ...) is *not* red anymore.
+void showErrorSnackBar(BuildContext context, String message,
+    {Duration? duration}) {
+  final scheme = Theme.of(context).colorScheme;
+  ScaffoldMessenger.of(context).showSnackBar(
+    SnackBar(
+      backgroundColor: scheme.error,
+      duration: duration ?? const Duration(seconds: 4),
+      content: Text(message, style: TextStyle(color: scheme.onError)),
+    ),
+  );
+}