Add frontend waitlist support for events

[miiiyyi]

Closes #2091
Summary
This PR adds frontend support for event waitlisting in Clark.

What changed

• added waitlist controls to event create and edit pages
• added waitlist_enabled and waitlist_size to the create/update event payloads
• added a joinWaitlistForSCEvent API helper for POST /events/:id/waitlist
• updated the event popup CTA logic to:
  • show Register when spots are available
  • show Join waitlist when the event is full and waitlist is enabled
  • stop offering normal registration for full events without a waitlist
• added frontend validation for invalid waitlist sizes

Notes

• current frontend only knows waitlist_enabled and waitlist_size
• waitlist occupancy/count is not currently exposed by SCEvents, so Clark cannot yet show exact remaining waitlist spots or disable the button ahead of time when the waitlist is already full. a follow-up update could expose waitlist occupancy/count from SCEvents so Clark can show clearer UI like waitlist spots: 2 instead of just waitlist available, and disable the Join waitlist button ahead of time when the waitlist is already full instead of only showing an error after the user clicks it

Screenshots:
create event with waitlist enabled:

create event with waitlist disabled:

user trying to register for an event that is full but waitlist is enabled:

user trying to register for an event that is full but waitlist is disabled:

user trying to join a waitlist that is full: