feat(player): 未就绪改由进度条展示加载态

test/widget_test.dart

@@ -185,7 +185,8 @@ void main() {
     await tester.pumpAndSettle();
     await _openTheaterPlayer(tester);
 
-    expect(find.byType(Slider), findsOneWidget);
+    expect(find.byKey(const Key('progress-loading')), findsOneWidget);
+    expect(find.byType(Slider), findsNothing);
     await tester.drag(find.byType(PageView), const Offset(0, -500));
     await tester.pumpAndSettle();
     expect(