fix(web): align outline close button styling

[techotaku39]

Summary

• style the conversation outline close action with the shared outline button variant
• match the close action's size, border, corner radius, and hover treatment to the adjacent load-earlier action
• keep the existing accessible label, tooltip, and close behavior unchanged
• add focused coverage for the close button's visible outline styling

Testing

• bun typecheck
• bun run test:web
• bun run build:web
• git diff --check
• manually verified on the mobile layout at port 3016

[github-actions]

Findings
No findings.

Questions
None.

Summary
Review mode: initial
No issues found in the modified lines. Residual risk: this is a visual styling change; no browser screenshot check was run in this automation pass.

Testing
Not run (automation)

HAPI Bot

[techotaku39]

@tiann 大佬好，这个 PR 目前 GitHub 显示可以干净合并，所有 CI 检查均已通过，HAPI Bot 审查也未发现问题。

这个 PR 统一了会话大纲顶部操作按钮的视觉样式：将关闭按钮改为与旁边的“加载更早”按钮一致的描边样式，并统一尺寸、圆角和悬停效果，同时保留原有的关闭行为、无障碍标签和提示信息。相关回归测试已经补充，也已在移动端测试环境中实际验证通过。

方便的时候能否帮忙审查合并一下？谢谢！