refactor: Move widgets to lib

lib/widgets/fluffy_banner.dart

@@ -0,0 +1,10 @@
+import 'package:flutter/material.dart';
+
+class FluffyBanner extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Image.asset(Theme.of(context).brightness == Brightness.dark
+        ? 'assets/banner_dark.png'
+        : 'assets/banner.png');
+  }
+}