feat(settings): add custom domain settings for share and direct links

[yabo083]

Description / 描述

Add two new global settings to allow users to configure custom domains for share links and direct links:

• share_url_domain: Custom domain for share links
• direct_link_url_domain: Custom domain for direct links

添加两个新的全局设置，允许用户为分享链接和直链配置自定义域名：

• share_url_domain：分享链接的自定义域名
• direct_link_url_domain：直链的自定义域名

Motivation and Context / 背景

When OpenList is deployed behind a reverse proxy or accessed via different domains (internal vs external), users may want share links and direct links to use a public domain instead of the internal IP address.

For example:

• Internal access: http://192.168.0.107:5244
• Public share links: https://share.example.com

This allows users to share links that work externally without exposing internal network addresses.

当 OpenList 部署在反向代理后面或通过不同域名访问时（内网 vs 外网），用户可能希望分享链接和直链使用公网域名而非内网IP。

例如：

• 内网访问：http://192.168.0.107:5244
• 公网分享链接：https://share.example.com

这允许用户分享可以从外部访问的链接，而无需暴露内部网络地址。

How Has This Been Tested? / 测试

• Configured both settings with custom domains
• Verified share links use the configured share_url_domain
• Verified direct links use the configured direct_link_url_domain
• Confirmed empty settings fall back to default behavior (using location.origin)

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）。
  Labels needed: enhancement, Module: Settings
• 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 feat: improve share page link generation with smart routing OpenList-Frontend#361
  • OpenList-Docs #XXXX

[hshpy]

反代配置X-Forwarded-Host或在config.json配置site_url