fix(ui): 修复媒体网格滚动和空状态显示问题

- 移除媒体网格的最大高度限制和自动垂直滚动
- 将媒体空状态占位符从网格布局改为弹性布局
- 添加媒体空状态占位符的文字居中和行高设置
- 修复API中任务ID类型转换问题

apps/web/src/styles/playground.css

@@ -1377,10 +1377,8 @@
   gap: 2px;
   width: 100%;
   max-width: var(--media-grid-max-width, 100%);
-  max-height: 600px;
   justify-self: center;
-  overflow-y: auto;
-  overscroll-behavior: contain;
+  overflow: visible;
 }
 
 .mediaTile {
@@ -1416,14 +1414,18 @@
 .mediaEmptyTile {
   position: absolute;
   inset: 0;
-  display: grid;
-  place-items: center;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
   gap: 10px;
   overflow: hidden;
   background: linear-gradient(110deg, #edf0f2 0%, #f7f8f9 42%, #e6eaee 78%);
   background-size: 220% 100%;
   color: #6b7280;
   font-size: var(--font-size-sm);
+  line-height: 1.4;
+  text-align: center;
   animation: mediaLoadingShimmer 1.35s ease-in-out infinite;
 }