Add FileTree theme component with collapsible grid UI and new icons

[Copilot]

Summary

Added a BEM-styled FileTree theme component with safe link handling, grid-driven collapse/expand like Sidebar, and new folder/file SVG icons exported through theme icons/index for consistent usage. Keyboard navigation (arrow keys, focus management) and break-word labels ensure accessibility and readability.

• Component: New FileTree React component with recursive rendering, grid 1fr/0fr children animation, arrow-toggle, and safe href guarding.
• Styling: .scss using existing theme variables and BEM blocks/elements; label overflow uses overflow-wrap: break-word.
• Assets/Exports: Added folder/file SVGs to theme assets, re-exported in icons.ts, and surfaced FileTree from theme/index.
• Keyboard/Focus: Arrow keys expand/collapse and move between items; first item is focusable, others receive focus via navigation.

Example usage:

<FileTree
  items={[
    { name: 'src', children: [{ name: 'index.ts', href: '/index' }] },
    { name: 'README.md', href: '/readme' },
  ]}
/>

https://github.com/user-attachments/assets/29aafc91-33dd-4bf3-aea9-c5a033dda62c

Related Issue

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

Original prompt

试着增加 core/theme/components/FileTree 组件

要求风格

1. 使用 .scss，BEM，与其他组件保持风格一致
2. 不要增加任何新的 dependencies
3. 在 theme/assets 中增加 icon，一个文件夹的 svg，一个文件的 svg
4. 文件夹展开的 UI 效果，仿照 Sidebar 组件中使用 grid + 1fr/0fr 切换

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