Restore fnm availability after suitup rewrites shell config

[Copilot]

• Re-check the reviewer comment and confirm the remaining requested changes
• Re-inspect the setup, frontend installer, and install.sh flows before editing
• Fix fnm installation success/failure handling and add Homebrew fallback when available
• Detect existing suitup-managed state and avoid preselecting already-completed setup steps
• Add an init vs append choice to install.sh and cover it with focused tests
• Run targeted tests for the touched areas, review the final diff, and update the PR title/comment reply

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[ChangeHow]

@copilot 找到问题了，是因为安装失败了，你对安装失败判断不对啊：
┌ Suit up!
│
◇ Select setup steps:
│ Frontend Tools
│
◇ Starting installation...
│
◇ Installing fnm...
curl: (22) The requested URL returned error: 504
│
◆ fnm installed

失败了怎么还是 installed？失败了就尝试 brew，如果用户前面选择了 brew/或者本地有 brew。

完成改造之后帮我做下面三件事情：

1. 更新最初始的选择，最好增加一个检测本地的 suitup 配置，根据已经设置完成的内容，把默认勾选的选项给反选上。
2. curl 安装方式最前面增加 init 还是 append 模式，append 模式就是现在 cli 提供的增量脚本
3. 完成上述所有任务之后，review 改动内容，然后更新合适的 PR 标题

[Copilot]

@copilot 找到问题了，是因为安装失败了，你对安装失败判断不对啊：
┌ Suit up!
│
...

已处理，提交在 ec58484。

现在 curl 安装 fnm 失败时不会再误报 success；如果本机有 brew，会自动 fallback 到 brew install fnm，否则会明确 warning 并跳过后续基于 fnm 的 Node 安装。setup 也会根据本地已完成的 suitup 配置/前端工具状态默认反选对应步骤，install.sh 现在会先让用户选 init 或 append。

UI 截图可用这个链接： https://github.com/user-attachments/assets/f62fbda0-3704-4771-9d64-d6e6791f4cf7

我这边建议的 PR 标题更新为：Improve fnm install recovery and installer rerun defaults