Re-enable the Newsletter with a real email capture

[Sealjay]

Context

In feat/design-review-overhaul the Newsletter was hidden but NOT deleted: it was removed from the nav (Base.astro), the homepage realm card (index.astro), and the About CTA (about.astro), and the castle scroll hotspot was repurposed as the Podcast/Listen hotspot (CastleSceneRetro.astro → /#listen). src/pages/newsletter/index.astro remains as a LIVE but UNLINKED route so nothing 404s, and the existing RSS autodiscovery in Base.astro is untouched. The freed nav/castle slot is currently the Podcast.

Goal

Re-enable the newsletter with a real, working email-capture flow and re-link it from the site.

Tasks

• Choose a privacy-friendly capture provider (e.g. Buttondown / MailerLite / Listmonk) with GDPR consent + double opt-in.
• Replace the 'Coming Soon' stub in src/pages/newsletter/index.astro with a real, accessible form (labelled input, error/success states, WCAG AA, tokens only — no hardcoded hex).
• Wire submission + confirmation (and unsubscribe).
• Decide the IA: restore a nav slot (alongside or in place of Listen), the homepage realm card, and the About CTA; decide whether the castle keeps the Podcast scroll or regains a newsletter hotspot.
• If a newsletter RSS feed exists, add it to autodiscovery in Base.astro.
• Remove the temporary 'unlinked route' status; ensure no dead links.

Acceptance

Working capture + confirmation, page re-linked from nav/home/About, build green (bun run build), AA contrast and keyboard support verified.