[Frontend] — New `/creator-events/join` page (invite code redemption)

[Olowodarey]

Background

CreateEventForm.tsx already builds a share URL pointing at this
not-yet-existent route:

But there is no src/app/(authenticated)/creator-events/join/page.tsx —
visiting that URL today 404s. The only existing invite-code entry point is
the inline input on [id]/page.tsx (lines 283-297), which requires already
knowing the event's id (URL segment) — useless for someone who only has an
invite code (the whole point of sharing a code instead of a link to an
unlisted event).

Goal

Add /creator-events/join — a page that accepts ?code=XXXX (or lets the
user type a code), looks up the event via getEventByCode, shows a preview
(via InvitePreview if suitable), and lets the user join.

Acceptance Criteria

• Navigating to /creator-events/join?code=APOLLO-2026 auto-looks-up
• Navigating to /creator-events/join (no query param) shows an empty
• An invalid/unknown code shows a not-found message, no crash
• A cancelled event's code shows the "cancelled" message
• Joining successfully redirects to /creator-events/event-001