Airdrop: move participant count from hero to Leaderboard section

[realproject7]

Problem

"3 participants earning" is shown at the bottom of the CampaignHero. It's not very useful there and adds clutter. Better placement is in the Leaderboard section where participant context is more relevant.

Solution

1. Remove participant count from `CampaignHero.tsx`
2. Add participant count to `Leaderboard.tsx` — show "X participants" as a subtitle below the section title

Leaderboard section header

Before:
```
Leaderboard
```

After:
```
Leaderboard
3 participants
```

The count comes from the existing `/api/airdrop/status` response (`totalParticipants`), or can be derived from the leaderboard data length.

Files

• `src/components/airdrop/CampaignHero.tsx` — remove participant count section
• `src/components/airdrop/Leaderboard.tsx` — add participant count subtitle

Acceptance Criteria

• Participant count removed from hero
• Participant count shown below Leaderboard title
• Format: just "X participants" (not "X participants earning")