feat: procedural season background with drifting petals

test/flutter_test_config.dart

@@ -0,0 +1,9 @@
+import 'dart:async';
+
+import 'package:block_seasons/ui/widgets/season_background.dart';
+
+Future<void> testExecutable(FutureOr<void> Function() testMain) async {
+  // Looping ambience animations never settle under pumpAndSettle.
+  debugDisableLoopingAnimations = true;
+  await testMain();
+}