feat(workspace): add file tree preview panel

[musnows]

Summary

• Add a right-side workspace file tree opened from the top-right toolbar.
• Add text file preview with multiple tabs, close buttons, source/render toggle for Markdown, raw HTML Markdown rendering, relative image loading, and unsupported-file fallback.
• Add file/folder context menu actions for adding references, copying absolute/relative paths, and opening files in Finder/file manager.
• Show referenced files on sent user messages so the visible prompt keeps its file context.
• Keep the file tree panel title aligned with the active workspace folder name.

Validation

• npm run typecheck
• npm test -- src/renderer/src/components/chat/ChatFileTreePanel.test.ts src/renderer/src/lib/workspace-text-preview.test.ts src/renderer/src/components/chat/tests/WorkspaceModeTabs.test.ts src/main/services/workspace-service.test.ts
• npm test -- src/renderer/src/agent/kun-runtime.test.ts src/renderer/src/agent/kun-mapper.test.ts src/renderer/src/components/chat/MessageTimeline.tool-summary.test.ts src/renderer/src/store/chat-store-thread-actions.test.ts
• npm run build
• npm run lint (passes with existing 9 react-hooks warnings)
• npm run dist
• Packaged macOS arm64 App smoke test: opened /tmp/kun-gui-file-tree-test/Kun.app, verified the file button is the rightmost topbar button, the side file tree opens, the title shows default_workspace, and files are listed.

中文说明

• 在右上角工具栏增加文件树入口，并以右侧栏显示工作区文件树。
• 支持文本文件多标签预览、关闭标签、Markdown 源码/渲染切换、Markdown 内 HTML 渲染、相对图片加载，以及非文本文件不支持预览提示。
• 文件/文件夹右键菜单支持添加引用、复制绝对/相对路径、在 Finder/文件管理器中显示。
• 发送后的用户消息会显示本次对话引用了哪些文件，避免只看到 prompt 而丢失上下文。
• 文件树面板标题显示当前工作区目录名，不再固定显示“文件”。

Fixes #258

Issue: #258

Latest package for manual verification

• Rebuilt package from commit 709176a with npm run dist.
• Apple Silicon zip: dist/Kun-0.1.0-mac-arm64.zip
• Apple Silicon app extracted for local testing: dist/Kun.app
• Intel Mac zip: dist/Kun-0.1.0-mac-x64.zip
• User smoke test reported no obvious issues after the latest file-reference display update.

最新手动验收包

• 已基于 commit 709176a 重新执行 npm run dist 打包。
• Apple Silicon zip：dist/Kun-0.1.0-mac-arm64.zip
• 已解压可直接测试的 App：dist/Kun.app
• Intel Mac zip：dist/Kun-0.1.0-mac-x64.zip
• 用户已完成最新文件引用显示逻辑的手动冒烟测试，暂未发现明显问题。

[musnows]

新增文件树视图，能够看到文件，并添加到对话

对话引用的文件支持显示（之前只会显示prompt，看不到引用的文件）