[DwC export]: Add Download Archive button in Export Package form

[grantfitzsimmons]

Goal

Add a Download button inside the Export Package form dialog that triggers an immediate download of the DwCA for that package.

Background

Admins need a way to manually trigger a one-off export without waiting for the RSS schedule. The download should reflect the current state of the data.

Acceptance Criteria

• A Download button is visible in the Export Package form dialog.
• Clicking it triggers generation of the DwCA and initiates a browser download.
• A progress indicator is shown while the archive is being built.
• The download is available for any saved Export Package (not just ones with RSS enabled).