fix(markdown): preview local/relative images in .md files

The shared MarkdownText only renders absolute http(s) images (relative
links are disabled for chat security), so a local image referenced by a
previewed markdown file - ![alt](./img.png), an absolute /cwd/img.png,
or a reference definition - showed only its alt text.

Rewrite such destinations into absolute /sidebar/file media URLs (with
the GUI origin prefix) so MarkdownText accepts them and the host media
route serves the bytes (still restricted to the session cwd). The
rewriter is a dependency-free pure helper (markdown-images.ts) with a
focused unit spec; TextEditor passes the rewritten preview text to both
the plain MarkdownText and the mermaid-lazy preview.

README.md

@@ -106,6 +106,7 @@
 
 - 🔧 **xterm 依赖迁移**：弃用的 xterm 迁移至 `@xterm/xterm`（Closes [#122](https://github.com/omdsh-dev/DSH-better-sidebar/issues/122)，[#128](https://github.com/omdsh-dev/DSH-better-sidebar/pull/128)）
 - 📝 **Markdown 编辑器**：选区转对话弹窗恢复可用（[#24](https://github.com/omdsh-dev/DSH-better-sidebar/pull/24)）
+- 🖼️ **Markdown 预览支持本地/相对路径图片**：预览 `.md` 时把指向本地文件的图片目标（相对/绝对路径、引用式 `[id]: url`）重写为 `/sidebar/file` 媒体 URL 并显示（此前仅绝对 http(s) 图片能渲染，相对路径只显示 alt 文本）
 - 🐛 **node-pty 加载失败不再拖垮 server**（[#140](https://github.com/omdsh-dev/DSH-better-sidebar/issues/140)）：宿主半改为懒加载 node-pty，缺失时插件照常挂载，终端以修复提示横幅（可复制命令 + 重试按钮）呈现，agent 终端工具自动跳过
 - 🧪 测试工程：单元测试拆分（#141）+ smoke 偶发失败修复