add download button loading and outcome states

[sgerner]

This change makes the search-result download button provide immediate visual feedback while the initial network request is pending and after it resolves.

What changed:

• Added a per-result loading spinner and disabled state while POST /api/download is in flight.
• Added transient success and error button states after the request completes.
• Added translations for the new button labels.
• Extended the same spinner treatment to the downloads tab refresh, clear-completed, and per-job retry actions.

Why:

• The download flow was otherwise silent during the initial request, which made the UI feel unresponsive.
• A success or error state on the button gives immediate confirmation of the network result without relying only on toast notifications.

Impact:

• Users can tell at a glance whether the request is still pending, succeeded, or failed.
• The behavior is localized to the UI and does not change backend download handling.

Checks:

• Parsed the inline browser scripts successfully.
• Rebuilt the local librarr:prowlarr-audiobooks image.
• Recreated the running librarr container.

[JeremiahM37]

Looks good