Sandbox UI: Account Browser (Screen 1) on real GET /v1/accounts

[tiana-code]

Wire the first Sandbox UI screen to the live ledger backend.

Screen 1 Account Browser consuming GET /v1/accounts (PR #110) via a typed TanStack Query hook, replacing the mock-direct-read pattern.

Scope (matches verified AccountResponse = id, name, type, currency, status):

• useAccounts(page, size) TanStack Query hook over GET /v1/accounts?page=&size=
• Typed API client + contract types (no invented fields)
• /accounts route in Shell, sidebar Accounts navigable + active
• Accounts table: Account ID (mono), Name, Type pill, Currency, Status pill
• Server-authoritative pagination (page/size/totalPages/totalElements), Prev/Next
• loading / error+retry / empty states
• Vitest + RTL tests for all states and pagination

Out of scope: balance column, owner/avatar, created column, KPI strip, search, filters, page-size selector, real Keycloak login.

Spec: docs/plans/account-browser/spec.md