[ flox1an/nostube ] fix(zaps): update cached zap amount when switching videos

Changed useEventZaps to use useMemo instead of useState for the cached

value, so it recalculates when eventId changes. Previously the cached

value was only set once on mount, causing stale zap amounts to display

when navigating between videos.

Co-Authored-By: Claude Opus 4.5

https://github.com/flox1an/nostube/commit/9b2a0d824aae4c2676c2dc03e3ac6f4571f1ee99