feat(player): animate circular feedback on transport buttons

test/mornye_playback_button_test.dart

@@ -27,6 +27,18 @@ void main() {
     ),
   );
 
+  double feedbackOpacity(WidgetTester tester) {
+    final feedback = tester.widget<DecoratedBox>(
+      find
+          .ancestor(
+            of: find.byType(IconButton),
+            matching: find.byType(DecoratedBox),
+          )
+          .first,
+    );
+    return (feedback.decoration as BoxDecoration).color!.a;
+  }
+
   testWidgets('press feedback keeps the tap target fixed and cancels safely', (
     tester,
   ) async {
@@ -36,15 +48,18 @@ void main() {
     final icon = find.byIcon(CupertinoIcons.play_fill);
     final bounds = tester.getRect(button);
     final iconBounds = tester.getRect(icon);
+    expect(feedbackOpacity(tester), 0);
     final press = await tester.startGesture(bounds.center);
     await tester.pump(const Duration(milliseconds: 100));
     await tester.pump(const Duration(milliseconds: 200));
     expect(tester.getRect(icon).width, lessThan(iconBounds.width));
     expect(tester.getRect(button), bounds);
+    expect(feedbackOpacity(tester), closeTo(0.14, 0.001));
     await press.up();
     await tester.pumpAndSettle();
     expect(calls, 1);
     expect(tester.getRect(icon), iconBounds);
+    expect(feedbackOpacity(tester), 0);
 
     final cancelled = await tester.startGesture(bounds.center);
     await tester.pump(const Duration(milliseconds: 100));
@@ -52,8 +67,40 @@ void main() {
     await tester.pumpAndSettle();
     expect(calls, 1);
     expect(tester.getRect(icon), iconBounds);
+    expect(feedbackOpacity(tester), 0);
   });
 
+  for (final icon in [
+    CupertinoIcons.play_fill,
+    CupertinoIcons.pause_fill,
+    CupertinoIcons.forward_fill,
+    CupertinoIcons.backward_fill,
+  ]) {
+    testWidgets('quick tap on $icon fades a circular highlight in and out', (
+      tester,
+    ) async {
+      var calls = 0;
+      await pumpButton(tester, icon: icon, onPressed: () => calls++);
+      final button = find.byType(IconButton);
+      final bounds = tester.getRect(button);
+      await tester.tap(button);
+      expect(calls, 1);
+      await tester.pump();
+      await tester.pump(const Duration(milliseconds: 50));
+      final entering = feedbackOpacity(tester);
+      expect(entering, greaterThan(0));
+      await tester.pump(const Duration(milliseconds: 60));
+      final peak = feedbackOpacity(tester);
+      expect(peak, greaterThan(entering));
+      await tester.pump(const Duration(milliseconds: 220));
+      expect(feedbackOpacity(tester), lessThan(peak));
+      expect(tester.getRect(button), bounds);
+      await tester.pumpAndSettle();
+      expect(feedbackOpacity(tester), 0);
+      expect(calls, 1);
+    });
+  }
+
   for (final icon in [
     CupertinoIcons.forward_fill,
     CupertinoIcons.backward_fill,