fix(settings): 避免模型下拉展开时抽屉下方字段被顶开

[su-fen]

Linked issue

Closes #712

Summary

供应商配置自定义设置抽屉里，「标题生成模型 / Commit 备注生成模型 / 澄清对话模型」展开后，下方字段会轻微下移。

ModelPicker 的 DropdownMenu 是 fragment；菜单打开时 Base UI 会在 trigger 两侧插入 FocusGuard。Tailwind v4 的 space-y-* 会对非末子元素加 margin-bottom，于是 trigger 多出约 6px 间距。用一层包装把 FocusGuard 关在内部，外层 space-y 的子节点开合前后不变。记忆设置、Cron 提示词表单等同样使用该选择器的地方一并修好。

Change scope

• Modules: agent-ui
• Key paths:
  • crates/agent-ui/src/pages/settings/modelPicker.tsx

Screenshots / preview

截图由作者补充（展开前 / 展开后，确认下方字段不再位移）。

Verification

• 对照 Base UI Menu.Trigger 打开时注入 FocusGuard 的实现，以及 Tailwind v4 space-y-* 的 :not(:last-child) { margin-bottom } 选择器，确认位移来自 spacing 上下文被打断。
• 未新增自动化测试：该问题依赖真实菜单打开时的 sibling 结构，现有套件没有 ModelPicker 用例。

Pre-submit checklist

• A requirement issue is linked (or this is a trivial fix that needs no issue, as explained in the summary).
• Synced with the target branch; no merge conflicts.
• The change is focused, with no unrelated modifications.
• No secrets, tokens, or personal data included.
• Docs are updated for changes affecting user behavior, deployment, or configuration.

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[github-actions]

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