Add optional donation to registration; fix soldOutLines server crash

The register form now offers a "Donation amount" (suggested amounts,
custom, or none) under the ticket options. The donation is added to the
Yoco checkout as its own line and the webhook records it as a separate
donation payment linked to the registration payment.

Move soldOutLines() out of the "use client" JoinWaitlistButton module
into lib/waitlist.ts so the server-rendered event page can call it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project follows [Semantic Versioning](https://semver.org/).
 
 ## [Unreleased]
 
+### Added
+
+- The register form asks for an optional **Donation amount** under the ticket options. Pick a suggested amount (worked out from the ticket total), enter a custom amount (min R15), or choose None. The donation shows in the total and is paid in the same Yoco checkout as the tickets. It's recorded as a separate donation to the event, so it never counts towards the registration's balance.
+
+### Fixed
+
+- Event pages with a waitlist crashed with "Attempted to call soldOutLines() from the server". The sold-out helper now lives in `lib/waitlist.ts` so the server page can call it.
+
 ## [1.29.1] - 2026-10-21
 
 ### Changed