fix(#1341): 移除 useStep6Cover 未使用的 useRef 导入，修复 Docker 构建 tsc TS6133 错误

apps/web/src/pages/generate/hooks/useStep6Cover.ts

@@ -2,7 +2,7 @@
  * Step 6 封面设置 Hook
  * 封装封面设置的交互逻辑，对接后端封面模板 CRUD API
  */
-import { useCallback, useEffect, useRef, useState } from "react"
+import { useCallback, useEffect, useState } from "react"
 import { message } from "antd"
 import type { CoverConfig, CoverTemplate } from "../../editing-planner/types"
 import { generateCover } from "@/api/template-editor"