Add server-side cursor pagination and incremental polling

[schjonhaug]

Parent: #236 (Phase 2)

Problem

GET /api/wallets/{checksum}/detail returns up to 10,000 transactions with no pagination. Users with 10K+ transactions silently lose visibility of older history. Every 60-second poll re-downloads the full payload (~5.5 MB at 10K tx).

What to change

Server-side cursor pagination

• Add cursor and page_size query parameters to the detail endpoint
• Paginate on (confirmed_at, txid) composite cursor
• Default page size: 50–100 transactions
• Return next_cursor in response for the frontend to fetch more

Incremental polling

• Add since_timestamp parameter to fetch only new/changed transactions since last poll
• Frontend sends last-known timestamp, API returns only the delta
• Dramatically reduces bandwidth on subsequent polls

Key files

• backend/src/handlers/wallet.rs — detail endpoint
• backend/src/metadata/transaction.rs — query with pagination
• frontend/src/hooks/useWalletDetail.ts — polling logic
• frontend/src/components/transactions.tsx — infinite scroll / load more