fix(fs/storage-details): use used space

[KirCute]

Description / 描述

驱动空间使用情况，改为使用总空间和已用空间计算进度条百分比/显示文本

Motivation and Context / 背景

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).
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• 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 feat(drivers): support getting disk usage of some drivers OpenList#1905

[Copilot]

Pull request overview

This PR updates the storage usage calculation logic to use a new used_space field directly from the backend instead of computing it as total_space - free_space. This change simplifies the calculation and allows the backend to provide the authoritative used space value.

Key changes:

• Added used_space optional field to MountDetails type
• Updated toReadableUsage to use used_space directly
• Enhanced usedPercentage with additional null checks and edge case handling
• Improved showDiskUsage with more robust null checking

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/types/obj.ts	Adds used_space optional field to MountDetails type to support direct used space reporting from backend
src/utils/storage.ts	Updates storage calculation functions to use used_space field with improved validation and edge case handling

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