Add account-free temporary preview publishing

[huangdun]

Summary

• add an explicit Temporary preview action beside Download Code for completed previews
• publish the selected HTML only after a user click, then copy the public URL
• support stable URL updates plus open, copy, and revoke controls
• persist only the project-scoped update capability in local storage
• disclose the 7-day lifetime, public-link visibility, and third-party temp.md hosting in the UI
• recover from expired or revoked capabilities by creating a fresh preview

Privacy and service disclosure

I maintain temp.md, the third-party service used by this integration. No content is uploaded until the user explicitly clicks Publish preview. Published previews are public to anyone with the URL and expire after seven days. Account, API-key, repository, and deployment setup are not required.

The narrowly scoped browser CORS prerequisite has been merged separately in https://github.com/huangdun/tempmd/pull/2; its production rollout is pending.

Verification

• Jest: 50 passed, 6 skipped
• TypeScript typecheck passed
• Vite production build passed
• ESLint passed for every changed TypeScript file
• added unit coverage for create, update, revoke, error handling, stale capabilities, and per-project persistence
• extended the existing optional QA flow for the toolbar action

The repository-wide lint command still reports existing violations in untouched files.

Closes #623