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/pages/ProjectPage.tsx

@@ -1,10 +1,11 @@
 import { useEffect, useState } from 'react'
-import { Link, useParams } from 'react-router-dom'
+import { Link, useNavigate, useParams } from 'react-router-dom'
 import { useProjectStore } from '../stores/project'
 import { getTasks } from '../api/project'
 import type { Task } from '../api/types'
 import StyleSelector from '../components/StyleSelector'
 import Skeleton from '../components/Skeleton'
+import EmptyState from '../components/EmptyState'
 
 const STATUS_LABELS: Record<string, { label: string; color: string }> = {
   pending: { label: '等待中', color: 'var(--text-muted)' },
@@ -15,6 +16,7 @@ const STATUS_LABELS: Record<string, { label: string; color: string }> = {
 
 export default function ProjectPage() {
   const { projectId = 'proj-default' } = useParams()
+  const navigate = useNavigate()
   const {
     name,
     style,
@@ -95,9 +97,11 @@ export default function ProjectPage() {
         </div>
 
         {tasks.length === 0 ? (
-          <p style={{ color: 'var(--text-muted)', textAlign: 'center', padding: 32 }}>
-            暂无任务
-          </p>
+          <EmptyState
+            title="暂无任务"
+            description="点击「新建生成」开始创建你的第一个游戏素材"
+            action={{ label: '新建生成', onClick: () => navigate(`/projects/${projectId}/generate`) }}
+          />
         ) : (
           <table style={{ width: '100%', borderCollapse: 'collapse' }}>
             <thead>