feat(123pan): add offline download

[mcxiedidi]

Description / 描述

• 新增123网盘离线下载实现(注：不是123_open)
• 添加相关API接口和常量配置
• 在路由和工具集中集成123网盘支持

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).
  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 feat(offline_download/123): add temp dir OpenList-Frontend#354
  • OpenList-Docs #XXXX

[Copilot]

Pull request overview

This PR adds offline download functionality for 123网盘 (123Pan) cloud storage service, complementing the existing 123_open implementation. The implementation follows the established patterns from other storage-backed offline download tools like 115 Cloud and PikPak.

Key Changes:

• Implements 123Pan offline download client with support for task submission, status tracking, and removal
• Adds API endpoints for resolving, submitting, listing, and deleting offline tasks in the 123 driver
• Integrates 123Pan into the offline download tool registry and routing system

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
server/router.go	Adds POST endpoint /set_123_pan for 123Pan offline download configuration
server/handles/offline_download.go	Implements Set123Pan handler for configuring 123Pan temp directory and initializing the tool
internal/offline_download/tool/download.go	Updates Transfer() to include "123Pan" in the list of tools that support direct transfer
internal/offline_download/tool/add.go	Adds logic to handle temp directory selection for 123Pan offline downloads
internal/offline_download/all.go	Registers the 123 offline download package via blank import
internal/offline_download/123/client.go	Implements the complete offline download client including AddURL, Remove, Status, and IsReady methods
internal/conf/const.go	Adds Pan123TempDir constant for storing the 123Pan temporary directory configuration
drivers/123/util.go	Adds API endpoints and implements OfflineDownload, GetOfflineTask, GetOfflineDownloadStatus, and DeleteOfflineTasks methods with supporting data structures

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