feat(player): 未就绪改由进度条展示加载态

test/player/player_video_surface_test.dart

@@ -12,8 +12,9 @@ void main() {
   ) async {
     await _pumpSurface(tester);
 
-    expect(find.byKey(const Key('player-fallback-background')), findsOneWidget);
-    expect(find.byKey(const Key('player-fallback-cover')), findsOneWidget);
+    expect(_fallbackColor(tester), Colors.black);
+    expect(find.byKey(const Key('player-fallback-cover')), findsNothing);
+    expect(find.byKey(const Key('player-fallback-scrim')), findsNothing);
     expect(_opacity(tester), 0);
     expect(find.byKey(const Key('player-debug-video')), findsNothing);
   });
@@ -48,7 +49,8 @@ void main() {
     expect(halfway, lessThan(1));
     await tester.pump(const Duration(milliseconds: 100));
     expect(_fadeValue(tester), 1);
-    expect(find.byKey(const Key('player-fallback-background')), findsOneWidget);
+    expect(_fallbackColor(tester), Colors.black);
+    expect(find.byKey(const Key('player-fallback-cover')), findsNothing);
     expect(find.byKey(const Key('player-debug-video')), findsNothing);
   });
 
@@ -62,7 +64,8 @@ void main() {
 
     expect(find.byKey(const Key('player-video-error')), findsOneWidget);
     expect(find.text('No route to host'), findsOneWidget);
-    expect(find.byKey(const Key('player-fallback-background')), findsOneWidget);
+    expect(_fallbackColor(tester), Colors.black);
+    expect(find.byKey(const Key('player-fallback-cover')), findsNothing);
     expect(_opacity(tester), 0);
     expect(find.byKey(const Key('player-debug-video')), findsNothing);
   });
@@ -82,7 +85,6 @@ Future<VideoPlayerController> _pumpSurface(
       home: Scaffold(
         body: PlayerVideoSurface(
           controller: controller,
-          posterUrl: '/files/poster.jpg',
           videoUrl: controller.dataSource,
           theater: false,
           onTap: () {},
@@ -94,6 +96,12 @@ Future<VideoPlayerController> _pumpSurface(
   return controller;
 }
 
+Color _fallbackColor(WidgetTester tester) {
+  return tester
+      .widget<ColoredBox>(find.byKey(const Key('player-fallback-background')))
+      .color;
+}
+
 double _opacity(WidgetTester tester) {
   return tester
       .widget<AnimatedOpacity>(find.byKey(const Key('player-video-opacity')))