[Airdrop P3] Weekly snapshots component

[realproject7]

Parent: #877
Depends on: #885

Scope

Weekly campaign recap cards (Section 4).

Design

┌─────────────────────────────────────────────────┐
│  Week 8 Recap (Apr 14-20)                       │
│    New stories: 12  ·  Token buys: 340          │
│    New referrals: 28                            │
│    MCap: $45,000 → $52,000 (+15.6%)            │
│    PL earned this week: 4,200                   │
└─────────────────────────────────────────────────┘

• Scrollable list of weekly recaps
• Latest week at top
• Each card: new stories, token buys, new referrals, mcap change (absolute + %), PL earned
• Empty state: "No weekly snapshots yet — check back after the first week!"

Data Source

GET /api/airdrop/snapshots

Files

• src/components/airdrop/WeeklySnapshots.tsx

Acceptance Criteria

• Displays weekly recap cards in reverse chronological order
• Each card shows all stats (stories, buys, referrals, mcap change, PL earned)
• MCap change shows both absolute values and percentage
• Handles empty state gracefully
• Responsive layout