[Feature]: Use Spinner on meetup loading states

[FrankezeCode]

Problem or opportunity

Several pages show plain “Loading…” text while data fetches. The profile page already uses the shared <Spinner /> component — meetup and check-in pages should match for consistent UX and accessibility.

Proposed solution

• Replace text-only loaders on /meetup/[meetupId] with <Spinner /> (centered, with loading message)
• Replace loader on /meetup/[meetupId]/circles with <Spinner />
• Replace Suspense fallback on /checkin with <Spinner />
• Spinner uses existing src/components/ui/spinner.tsx (do not create a duplicate)
• Loading states remain accessible (role="status" or sr-only text)
• No unrelated file changes (no lockfile, no .nvmrc)

HOW TO TEST

1. Run npm run dev
2. Visit /meetup/[valid-slug] — confirm spinner shows briefly before content loads
3. Visit /meetup/[valid-slug]/circles — same spinner pattern
4. Visit /checkin — spinner shows in loading fallback
5. Run npm run build — passes with no type errors

Alternatives considered

No response