fix: download toggle without page reload + clear explanation page on Google admin login
1) Admin→Audio: the "download-for-email" toggle (⬇) was a form POST with redirect →
full page reload on every click (player + scroll reset). Now AJAX via the existing
/admin/audio/api/:id (accepts downloadable), button state updated in-place, no
reload. (Old POST route stays as an unused fallback.)
2) /auth/login did not show the query error (error:null) → message only appeared in
the URL. Now ?error= is rendered, and the "Google address belongs to an admin"
case gets its own clear explanation block (gerr=admin): why it is rejected + the
two paths (admin = password, fan = different Google account).
Co-Authored-By: Claude <noreply@…>