Sandbox UI: Transaction Explorer screen (Screen 3) on live GET /v1/transactions

[tiana-code]

Problem

Screen 3 (Transaction Explorer) of the Sandbox UI has no implementation. The backing endpoint GET /v1/transactions is now live (PR #118), so the screen can wire to the real paginated API.

Scope

A /transactions route listing posted transactions from GET /v1/transactions:

• Typed useTransactions(page, size) TanStack hook over the real endpoint (mirrors useAccounts).
• Table of the real TransactionResponse fields only: id (mono, prefixed ULID), reference, status pill, postedAt (formatted via formatInstant).
• Server-authoritative pagination (page/size/totalPages/totalElements), same controls as the Account Browser.
• Loading / error+retry / empty states via the shared StateMessage.
• No amount/currency/counterparty columns: those are not in the listing payload (they belong to the detail endpoint Add TransactionController get and reverse endpoints #105). Render only real server fields.
• Sidebar nav entry -> react-router Link.

Out of scope

• Transaction detail drill-in (waits on Add TransactionController get and reverse endpoints #105).
• Filters, page-size selector, URL query-param sync (backlog).

Acceptance

• /transactions renders a table of real fields with tx_-prefixed ids and a status pill.
• Pagination is server-authoritative; prev/next reflect totalPages.
• Loading, error+retry, and empty states render.
• Vitest+RTL tests with fetch mocked (no msw).