feat: add newsletter signup port

[glitchedmob]

Ports the newsletter signup JSON endpoint and Svelte components for the next migration layer.

• Adds /api/newsletter POST and GET handling with server-only provider config and the same { success } JSON response shape.
• Sends only trimmed name, email, and configured list_uuids to listmonk, while preserving honeypot success without upstream calls.
• Keeps the original form DOM contract, native email validation, SweetAlert copy, and no field clearing after success.
• Adds deliberate hardening for malformed payloads, invalid inputs, upstream non-2xx or bad JSON responses, and timed-out upstream requests.
• Ports the intro/email signup block with the hard-coded conference copy and app badge links.