feat: implement wallet balance API endpoint

[MerlinTheWhiz]

Closes #253

Changes made:

1. src/proxy.ts:24 — Added "/api/wallet" to PROTECTED_API_ROUTES so the middleware injects x-user-id header (and handles token refresh/fingerprint validation).
2. src/app/api/wallet/balance/route.ts — New GET endpoint that:

• Authenticates via x-user-id header
• Supports optional ?currency=NGN query filter
• Returns per-currency wallet data with:
  Field Description
  balance Raw wallet balance
  pendingWithdrawals Sum of gifts in confirmed status where user is sender (committed but not yet debited)
  lockedBalance Sum of gifts received (completed/sent status) with unlockDatetime in the future
  availableBalance balance - pendingWithdrawals - lockedBalance (floor 0)
  Example response:
  { "success": true, "data": { "wallets": [ { "id": "...", "currency": "NGN", "balance": 100000, "availableBalance": 95000, "pendingWithdrawals": 5000, "lockedBalance": 0 } ] } }

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