feat: 首页展示最近生成的素材画廊

- 后端新增 GET /api/v1/assets 接口，返回已完成任务的素材列表（按创建时间倒序）
- 前端新增 AssetGallery 组件，网格展示素材缩略图、类型标签和提示词
- HomePage 中工程列表上方展示最近生成素材

frontend/src/pages/HomePage.tsx

@@ -2,6 +2,7 @@ import { useEffect, useState } from 'react'
 import { useProjectListStore } from '../stores/projectList'
 import { useToastStore } from '../stores/toast'
 import ProjectCard from '../components/ProjectCard'
+import AssetGallery from '../components/AssetGallery'
 import CreateProjectModal from '../components/CreateProjectModal'
 import EmptyState from '../components/EmptyState'
 import Skeleton from '../components/Skeleton'
@@ -39,6 +40,8 @@ export default function HomePage() {
 
   return (
     <div className="container page-enter" style={{ paddingTop: 32, paddingBottom: 32 }}>
+      <AssetGallery />
+
       <div style={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center', marginBottom: 24 }}>
         <h1 style={{ fontSize: 22 }}>我的工程</h1>
         <button className="btn-primary" onClick={() => setModalOpen(true)}>