feat(ux): 空状态组件，替换各页面空白提示

新增 EmptyState 组件，ProjectPage 暂无任务时显示带操作按钮的
友好提示，AssetPreview 暂无素材时显示引导文案。

frontend/src/components/AssetPreview.tsx

@@ -1,4 +1,5 @@
 import type { Asset } from '../api/types'
+import EmptyState from './EmptyState'
 
 interface AssetPreviewProps {
   assets: Asset[]
@@ -6,11 +7,7 @@ interface AssetPreviewProps {
 
 export default function AssetPreview({ assets }: AssetPreviewProps) {
   if (assets.length === 0) {
-    return (
-      <div style={{ color: 'var(--text-muted)', textAlign: 'center', padding: 40 }}>
-        暂无素材
-      </div>
-    )
+    return <EmptyState title="暂无素材" description="生成完成后，素材将在这里展示" />
   }
 
   return (

frontend/src/components/EmptyState.tsx

@@ -0,0 +1,43 @@
+interface EmptyStateProps {
+  icon?: string
+  title: string
+  description?: string
+  action?: {
+    label: string
+    onClick: () => void
+  }
+}
+
+export default function EmptyState({ icon = '📭', title, description, action }: EmptyStateProps) {
+  return (
+    <div
+      style={{
+        display: 'flex',
+        flexDirection: 'column',
+        alignItems: 'center',
+        justifyContent: 'center',
+        padding: '48px 24px',
+        textAlign: 'center',
+      }}
+    >
+      <span style={{ fontSize: 48, marginBottom: 16, opacity: 0.6 }}>{icon}</span>
+      <h3 style={{ fontSize: 16, color: 'var(--text-secondary)', marginBottom: 8 }}>
+        {title}
+      </h3>
+      {description && (
+        <p style={{ fontSize: 13, color: 'var(--text-muted)', marginBottom: action ? 20 : 0 }}>
+          {description}
+        </p>
+      )}
+      {action && (
+        <button
+          className="btn-primary"
+          onClick={action.onClick}
+          style={{ padding: '8px 24px', fontSize: 13 }}
+        >
+          {action.label}
+        </button>
+      )}
+    </div>
+  )
+}