feat(chat): add delete action to dock file list rows

[chujieHong]

Each file row in the chat dock "modified files" panel now has a delete button (Trash2 icon, danger hover) to the left of the download button, with a Popconfirm guard since deletion is irreversible.

• Calls the existing DELETE /agents/{id}/workspace/file endpoint
• Deleted paths are hidden via a canonical-key set (the list itself is message-derived and would otherwise keep showing them)
• The file's open viewer tab is closed after deletion (same tab id function as openFileAt, so no mismatch)
• 404 during delete treats the file as already gone and hides the row
• Button styles shared with the download button; hover-reveal behavior extended to both actions

Summary

在专家聊天「已修改文件」面板的每行文件上，下载按钮左侧新增删除按钮（Trash2 图标、危险色 hover），点击弹 Popconfirm 二次确认后调用既有 DELETE /workspace/file 接口真实删除文件
删除后文件从列表消失（通过 canonical-key 集合过滤）、自动关闭该文件打开的查看器标签页；404 视为已删除并提示
改动 5 个前端文件：ChatDockFileList.tsx、ChatDockPanel.tsx、chatBrowserPanel.partial.less、中英文 locale

Target branch

• Base is develop (feature / fix — default)
• Base is main (release/* or hotfix/* only)

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactor / chore
• Release / hotfix

Test plan

• make all passes locally
• Added/updated tests

Checklist

• Updated CHANGELOG.md (if user-facing)
• README / docs updated (if needed)