feat: 结果页添加帧预览、GIF动画和导出选项

- 后端: GetAssets 返回素材级别 metadata，区分 frame/preview/spritesheet 类型
- 后端: 上传时区分素材类型标签，GIF URL 独立追踪
- 前端: 结果详情页默认展示拆分后的单帧网格
- 前端: 添加动作帧 GIF 预览弹窗
- 前端: 添加导出下拉菜单（整张精灵表/自动切分帧）
- 前端: 失败任务展示错误信息和重试次数

frontend/src/pages/ResultPage.tsx

@@ -79,26 +79,41 @@ export default function ResultPage() {
     )
   }
 
-  return (
-    <div className="container page-enter" style={{ paddingTop: 24, paddingBottom: 40 }}>
-      <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center', marginBottom: 24 }}>
-        <div style={{ display: 'flex', alignItems: 'center', gap: 16 }}>
-          <Link
-            to={`/projects/${projectId}`}
-            style={{ fontSize: 13, color: 'var(--text-secondary)' }}
-          >
+  if (task.status === 'failed') {
+    return (
+      <div className="container page-enter" style={{ paddingTop: 40 }}>
+        <div style={{ display: 'flex', alignItems: 'center', gap: 16, marginBottom: 24 }}>
+          <Link to={`/projects/${projectId}`} style={{ fontSize: 13, color: 'var(--text-secondary)' }}>
             &larr; 返回工程
           </Link>
-          <h1 style={{ fontSize: 22 }}>生成结果</h1>
+          <h1 style={{ fontSize: 22 }}>生成失败</h1>
         </div>
-        {assets.length > 0 && (
-          <button
-            className="btn-primary"
-            onClick={() => downloadAssets(assets)}
-            style={{ padding: '8px 20px', fontSize: 13 }}
-          >
-            下载全部素材
-          </button>
+        <div className="card" style={{ textAlign: 'center', padding: 40 }}>
+          <p style={{ color: 'var(--error)', marginBottom: 8 }}>
+            {task.error || '未知错误'}
+          </p>
+          <p style={{ fontSize: 13, color: 'var(--text-secondary)' }}>
+            重试次数: {task.retryCount ?? 0}
+          </p>
+        </div>
+      </div>
+    )
+  }
+
+  return (
+    <div className="container page-enter" style={{ paddingTop: 24, paddingBottom: 40 }}>
+      <div style={{ marginBottom: 24 }}>
+        <Link
+          to={`/projects/${projectId}`}
+          style={{ fontSize: 13, color: 'var(--text-secondary)' }}
+        >
+          &larr; 返回工程
+        </Link>
+        <h1 style={{ fontSize: 22, marginTop: 8 }}>生成结果</h1>
+        {task.prompt && (
+          <p style={{ fontSize: 13, color: 'var(--text-secondary)', marginTop: 4 }}>
+            提示词: {task.prompt}
+          </p>
         )}
       </div>
 
@@ -108,22 +123,3 @@ export default function ResultPage() {
     </div>
   )
 }
-
-async function downloadAssets(assets: Asset[]) {
-  for (const a of assets) {
-    try {
-      const res = await fetch(a.url)
-      const blob = await res.blob()
-      const blobUrl = URL.createObjectURL(blob)
-      const link = document.createElement('a')
-      link.href = blobUrl
-      link.download = `${a.id}.${a.format}`
-      document.body.appendChild(link)
-      link.click()
-      document.body.removeChild(link)
-      URL.revokeObjectURL(blobUrl)
-    } catch {
-      window.open(a.url, '_blank')
-    }
-  }
-}