v2.0: feat: no double-booking — switch between events at the same time

- Registering, starting a team or accepting a team invite is refused when it overlaps an event the
  student is registered or waitlisted for (SCHEDULE_CONFLICT with the clashing events)
- Switch: the student agrees to give up the overlapping places (only before those events start);
  they're cancelled through the normal cancel path (seats go to the waitlist, leaders withdraw their
  team) and the new registration goes ahead
- Event page warns in advance and offers "Switch to this event"; one dialog everywhere explains what
  switching gives up; team invite picker shows who's busy at that time
- Tests: scheduleClash (backend), ScheduleClash (frontend); test setup waits up to 5s so the parallel
  suite isn't flaky on a busy machine

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

v1.1: fix: club request progress line and the All tab

- The request progress uses the same stepper as events and recruitment (dots, connectors, labels),
  with amber "Changes requested" and red "Rejected" states at the step where they happened
- The All tab on club requests showed only the default tab's requests: it now uses an explicit ALL
  value, and the API treats status=ALL as no filter
- Removed the old pill-style stepper rules that turned the current step's text white

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

first version

feat: gallery as its own section with a Photo gallery button on events

The gallery moves off the event page into a Gallery section like Results:
/gallery lists events as cards (latest photos first, collage covers,
counts, search, All / With photos / To review) and /gallery/:eventId
shows the event's photos and videos, uploads and the review queue. The
event page keeps a single Photo gallery button with the photo count.
Notification and dashboard links now open the gallery page.

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