fix(pages/login): replace useFetch with useLoading

[xrgzs]

Description / 描述

Replace useFetch with useLoading for the login button.

Motivation and Context / 背景

useLoading (called without fetch=true) always calls setLoading(false) regardless of the response code, so the button correctly unblocks on any response including 401.

Fixes 0ef680c#commitcomment-183749887

Thanks to @Clouddark75 .

How Has This Been Tested? / 测试

Checklist / 检查清单

• I have read the CONTRIBUTING document.
  我已阅读 CONTRIBUTING 文档。
• I have formatted my code with go fmt or prettier.
  我已使用 go fmt 或 prettier 格式化提交的代码。
• I have added appropriate labels to this PR (or mentioned needed labels in the description if lacking permissions).
  我已为此 PR 添加了适当的标签（如无权限或需要的标签不存在，请在描述中说明，管理员将后续处理）。
• I have requested review from relevant code authors using the "Request review" feature when applicable.
  我已在适当情况下使用"Request review"功能请求相关代码作者进行审查。
• I have updated the repository accordingly (If it’s needed).
  我已相应更新了相关仓库（若适用）。
  • OpenList-Frontend #XXXX
  • OpenList-Docs #XXXX

[j2rong4cn]

我刚想提交pr