fix: close panel after stopping a pending ask_user prompt

[tsubasakong]

What

Fixes a panel state bug when the agent is waiting on an ask_user prompt.

When the user clicks stop while the panel is waiting for an answer:

• the agent now leaves the running state immediately,
• the pending question UI is cleared,
• the waiting promise is resolved cleanly,
• and the panel can be closed or reused normally.

Type

• Bug fix
• Feature / Improvement
• Refactor
• Documentation
• Website
• Demo / Testing
• Breaking change

Testing

• Tested in modern browsers
• No console errors
• Types/doc added

Local validation:

• npm ci
• npm run build -w @page-agent/ui
• npm run build -w page-agent (completed locally; the repo still prints existing package type-resolution diagnostics during build)
• git diff --check

Closes #183

Requirements / 要求

• I have read and follow the Code of Conduct and Contributing Guide . / 我已阅读并遵守行为准则。

Checklist

• This PR is NOT generated by a bot or AI agent acting autonomously. I have authored or meaningfully reviewed every change. / 此 PR 不是由 bot 或 AI 自主生成的，我已亲自编写或充分审查了每一处变更。

[CLAassistant]

All committers have signed the CLA.