docs: surface download popup cancel hint

[haosenwang1018]

Summary

• surface Enter confirm and Esc cancel directly in the download-provider popup title instead of relying only on the status-bar hint
• make the current download-provider picker flow easier to understand at the point of use
• continue issue Brilliant! Comments for improvements #344's cancellation/discoverability cleanup with a small in-product UX improvement

Testing

• cargo test -p llmfit --quiet
• git diff --check