Cleaning up Veggie Seasons (#2416)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

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https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
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[Discord]:
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https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

veggieseasons/lib/widgets/detail_buttons.dart

@@ -0,0 +1,148 @@
+// Copyright 2018 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui' as ui;
+
+import 'package:flutter/cupertino.dart';
+import '../styles.dart';
+
+/// Partially overlays and then blurs its child's background.
+class FrostedBox extends StatelessWidget {
+  const FrostedBox({
+    this.child,
+    super.key,
+  });
+
+  final Widget? child;
+
+  @override
+  Widget build(BuildContext context) {
+    return BackdropFilter(
+      filter: ui.ImageFilter.blur(sigmaX: 10, sigmaY: 10),
+      child: DecoratedBox(
+        decoration: const BoxDecoration(
+          color: Styles.frostedBackground,
+        ),
+        child: child,
+      ),
+    );
+  }
+}
+
+/// An Icon that implicitly animates changes to its color.
+class ColorChangingIcon extends ImplicitlyAnimatedWidget {
+  const ColorChangingIcon(
+    this.icon, {
+    this.color = CupertinoColors.black,
+    this.size,
+    required super.duration,
+    super.key,
+  });
+
+  final Color color;
+
+  final IconData icon;
+
+  final double? size;
+
+  @override
+  AnimatedWidgetBaseState<ColorChangingIcon> createState() =>
+      _ColorChangingIconState();
+}
+
+class _ColorChangingIconState
+    extends AnimatedWidgetBaseState<ColorChangingIcon> {
+  ColorTween? _colorTween;
+
+  @override
+  Widget build(BuildContext context) {
+    return Icon(
+      widget.icon,
+      semanticLabel: 'Close button',
+      size: widget.size,
+      color: _colorTween?.evaluate(animation),
+    );
+  }
+
+  @override
+  void forEachTween(TweenVisitor<dynamic> visitor) {
+    _colorTween = visitor(
+      _colorTween,
+      widget.color,
+      (dynamic value) => ColorTween(begin: value as Color?),
+    ) as ColorTween?;
+  }
+}
+
+/// A close button that invokes a callback when pressed.
+class CloseButton extends _DetailPageButton {
+  const CloseButton(VoidCallback onPressed, {super.key})
+      : super(onPressed, CupertinoIcons.chevron_back);
+}
+
+/// A share button that invokes a callback when pressed.
+class ShareButton extends _DetailPageButton {
+  const ShareButton(VoidCallback onPressed, {super.key})
+      : super(onPressed, CupertinoIcons.share);
+}
+
+/// A favorite button that invokes a callback when pressed.
+class FavoriteButton extends _DetailPageButton {
+  const FavoriteButton(VoidCallback onPressed, bool isFavorite, {super.key})
+      : super(
+          onPressed,
+          isFavorite ? CupertinoIcons.heart_fill : CupertinoIcons.heart,
+        );
+}
+
+class _DetailPageButton extends StatefulWidget {
+  const _DetailPageButton(this.onPressed, this.icon, {super.key});
+
+  final VoidCallback onPressed;
+  final IconData icon;
+
+  @override
+  State<_DetailPageButton> createState() => _DetailPageButtonState();
+}
+
+class _DetailPageButtonState extends State<_DetailPageButton> {
+  bool tapInProgress = false;
+
+  @override
+  Widget build(BuildContext context) {
+    return GestureDetector(
+      onTapDown: (details) {
+        setState(() => tapInProgress = true);
+      },
+      onTapUp: (details) {
+        setState(() => tapInProgress = false);
+        widget.onPressed();
+      },
+      onTapCancel: () {
+        setState(() => tapInProgress = false);
+      },
+      child: ClipOval(
+        child: FrostedBox(
+          child: Container(
+            width: 30,
+            height: 30,
+            decoration: BoxDecoration(
+              borderRadius: BorderRadius.circular(15),
+            ),
+            child: Center(
+              child: ColorChangingIcon(
+                widget.icon,
+                duration: const Duration(milliseconds: 300),
+                color: tapInProgress
+                    ? Styles.closeButtonPressed
+                    : Styles.closeButtonUnpressed,
+                size: 20,
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}