v5.3.2

Enable GitHub Pages on newsletter-assets repo so newsletter HTML renders
natively in the browser. Add PAGES_BASE constant and previewUrl to
image-host uploads, threaded through to Firestore campaign docs and the
Beehiiv fallback alert email. Restructure the fallback email asset links
to use clean anchor text with nested preview/raw links under "Full HTML"
instead of raw GitHub URLs. Update download-app-link email subject line.
Routine disposable-domains refresh.