fix(web): replace fs.cpSync to avoid Windows crash in extract step

[chx7776-afk]

Summary

Replaces the single fs.cpSync call in web/scripts/extract-content.ts with a small recursive copy helper.

Closes #573.

Why

On Windows with Node 22, fs.cpSync kills the process at the native layer with STATUS_STACK_BUFFER_OVERRUN (exit code 0xC0000409) and no JS-level error:

> web@0.1.0 extract
> tsx scripts/extract-content.ts

Extracting course content...
  Repo root: <repo root>
  Source: root chapter folders (17)

...then the process is gone. Because this script backs the predev hook, npm run dev never starts either.

The crash is not specific to this repo — copying a directory holding a single one-line text file is enough to trigger it, so there is no project-side workaround other than not calling fs.cpSync.

Change

fs.cpSync is used in exactly one place, copyChapterAssets(). The new copyDirRecursive() helper does the same thing via fs.readdirSync with withFileTypes + fs.mkdirSync + fs.copyFileSync, and recurses into subdirectories. path is already imported in this file.

No behaviour change on other platforms: same files land in the same destination layout.

Verification

Windows 11, Node v22.20.0, run against a clean tree (removed web/public/course-assets/ first):

	before	after
exit code	3221226505 (0xC0000409)	0
extraction output	aborted after Source: root chapter folders (17)	17 versions / 16 diffs / 51 docs
web/public/course-assets/	not written	74 files written

Also confirmed npm run build proceeds past the previously blocking predev step.

Notes

• One file touched, web/scripts/extract-content.ts: +21 / -1.
• No changes to chapter code or READMEs, so the three-language sync rule does not apply here.
• Opened with AI assistance for reproduction and drafting; I ran the before/after on my own machine and stand behind the change.

中文说明（Chinese summary）

摘要

把 web/scripts/extract-content.ts 中唯一一处 fs.cpSync 调用替换为一个小型递归复制函数。

关联 Issue：#573。

背景

在 Windows + Node 22 下，fs.cpSync 会在原生层直接终止进程，报 STATUS_STACK_BUFFER_OVERRUN（退出码 0xC0000409），且没有任何 JS 层报错：

> web@0.1.0 extract
> tsx scripts/extract-content.ts

Extracting course content...
  Repo root: <仓库根目录>
  Source: root chapter folders (17)

然后就没了下文。由于该脚本是 predev 钩子的实现，npm run dev 也随之无法启动。

这个崩溃与仓库本身无关 —— 即使只复制一个含单行文本文件的目录也照样触发，因此项目侧除了「不要调用 fs.cpSync」之外没有别的绕法。

改动

fs.cpSync 全仓库只用在一处，即 copyChapterAssets()。新增的 copyDirRecursive() 用 fs.readdirSync（带 withFileTypes）+ fs.mkdirSync + fs.copyFileSync 实现相同效果，并递归处理子目录。path 在该文件中已有引入。

其他平台行为不变：同样的文件落到同样的目标目录结构下。

验证

环境 Windows 11 + Node v22.20.0，在干净的工作区上运行（先删掉 web/public/course-assets/）：

	修复前	修复后
退出码	3221226505（0xC0000409）	0
提取输出	停在 Source: root chapter folders (17)	17 versions / 16 diffs / 51 docs
web/public/course-assets/	未生成	生成 74 个文件

同时确认 npm run build 能够越过此前阻塞它的 predev 步骤继续执行。

说明

• 仅改动一个文件 web/scripts/extract-content.ts：+21 / -1。
• 未改动任何章节代码或 README，因此不涉及三语同步规则。
• 本次修改借助 AI 完成复现与草拟；修复前后的对比由我本人在本机实测，并认可该改动。