[Airdrop] Farcaster referral code property name mismatch

[realproject7]

Parent: #877

Problem

Frontend sends useFarcaster: true but backend expects useFarcasterUsername: true when setting Farcaster username as referral code.

Frontend (src/components/airdrop/UserPoints.tsx):

body: JSON.stringify({ message, signature, useFarcaster: true })

Backend (src/app/api/airdrop/referral-code/route.ts):

useFarcasterUsername = body.useFarcasterUsername === true

Impact

The "Use Farcaster username" button silently fails — the request goes through but the backend never sees the flag, so it generates a nanoid instead of using the Farcaster username.

Fix

Change frontend to send useFarcasterUsername: true (matching the backend), or change backend to read useFarcaster (matching the frontend). Frontend fix is simpler.

Acceptance Criteria

• Property name matches between frontend and backend
• "Use Farcaster username" button correctly sets username as referral code