treewide: small refactors

lib/helpers/extensions/show_error_snack_bar.dart

@@ -0,0 +1,28 @@
+import "package:material_ui/material_ui.dart";
+
+extension ShowErrorSnackBar on ScaffoldMessengerState {
+  /// Shows [message] in a snackbar styled as an error.
+  ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showErrorSnackBar(
+    String message, {
+    ({String label, VoidCallback onPressed})? action,
+  }) {
+    final colorScheme = Theme.of(context).colorScheme;
+
+    return showSnackBar(
+      .new(
+        backgroundColor: colorScheme.errorContainer,
+        content: Text(
+          message,
+          style: .new(color: colorScheme.onErrorContainer),
+        ),
+        action: action == null
+            ? null
+            : .new(
+                label: action.label,
+                onPressed: action.onPressed,
+                textColor: colorScheme.onErrorContainer,
+              ),
+      ),
+    );
+  }
+}