fix(ui): 拆分媒体骨架底色，恢复可见度

test/theme/app_theme_test.dart

@@ -26,6 +26,8 @@ void main() {
     expect(AppColors.cardAlt, const Color(0xFF101010));
     expect(AppColors.raised, const Color(0xFF1C1C1E));
     expect(AppColors.border, const Color(0xFF1C1C1E));
+    expect(AppColors.skeletonSurface, const Color(0xFF0E0E0E));
+    expect(AppColors.skeletonMedia, const Color(0xFF1C1C1E));
     expect(AppColors.textPrimary, const Color(0xFFFFFFFF));
     expect(AppColors.textSecondary, const Color(0xFFB0B0B0));
     expect(AppColors.textTertiary, const Color(0xFF8E8E93));