Replace mock leaderboard with live Soroban RPC / indexer query (Closes #20)

[storm-beyndtech]

What

Replaces the 100 synthetic leaderboard rows with a live data source.

SorobanService.getLeaderboard(offset, limit, sortKey) returns one
{ entries, total } page from either:

1. Backend indexer — NEXT_PUBLIC_LEADERBOARD_API_URL (?offset&limit&sort),
   the intended source once smartdrop-backend /api/v1/leaderboard ships.
2. On-chain event scan (fallback) — aggregates lock_assets /
   unlock_assets / update_credits across the factory pools and
   NEXT_PUBLIC_POOL_CONTRACT_ID, mirroring the existing getPoolDepositors
   pattern. Best-effort over a ~7‑day lookback until the backend exists.

useLeaderboard now fetches only the current page (server-side sort), keeps the
30s refresh scoped to that page, and debounces the address filter at 300ms.

Acceptance criteria

• Removes synthetic data; fetches real addresses/credits/stake
• Server-side pagination (PAGE_SIZE = 10, offset/limit)
• Sort key passed to the query, not done in memory over a full set
• 300ms-debounced partial-address search on the current page
• 30s refresh re-fetches the current page only
• Existing Chakra Spinner / no-data states reused (page unchanged)

Files

• src/lib/soroban.ts
• src/hooks/useLeaderboard.ts

Notes for reviewers

• Type-checks clean (tsc) for both changed files.
• ⚠️ main currently has pre-existing build-breaking corruption unrelated to
  this PR — next.config.ts:4, src/app/farm/page.tsx:3, and
  ConnectWalletButton.tsx contain injected/garbled lines (see separate report).
  A red next build in CI stems from that, not from this change.