[UX] Implement Optimistic UI Updates for `RSVPButton.tsx`

[Aditya948351]

When a user clicks RSVP to an event, there is a delay while the server updates, leaving the UI feeling unresponsive. Update RSVPButton.tsx to instantly toggle state locally (optimistic update) and only revert if the Firebase mutation fails.

[gunjankushwaha17]

I'd like to work on this issue .

Proposed solution:
I'll update RSVPButton.tsx to flip its local state (e.g. via useState) immediately on click, before the Firebase write resolves, so the UI feels instant. The actual Firestore mutation will run in the background if it succeeds, nothing else happens; if it fails, I'll revert the local state back to its previous value and show a brief error/toast so the user knows their RSVP didn't go through. I'll also add a guard to prevent double-clicks from firing multiple mutations while one is in flight.

Could you please assign this to me?

[github-actions]

Hi @gunjankushwaha17! We have officially assigned this issue to you. Please start working on it, and good luck with your PR!

ETA: 3 days
Issue will be assigned to another contributor if failed to do so!

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[Aditya948351]

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