Admin tool: move money between any two accounts (with memo)

Admin-gated /admin/accounts (or /admin/transfers) tool to move money between any two accounts with a memo, via the SERVICE-scoped POST /api/v1/admin/transfers endpoint (sibling Treasury API tranche). Ledger-authoritative — no raw balance SQL.

• lib/treasury.ts: add adminTransfer({fromAccountId,toAccountId,amount,memo}).
• lib/actions/admin-*.ts: adminTransferAction — admin-gated, audited (explorer_audit, target_type account/transaction), surfaces the API error envelope.
• UI: pick source + destination account (search by id/name via existing account DAL), amount, memo; confirm step for large amounts; show the resulting txn id.
• Reuses the lib/treasury.ts client + admin gate from PAR-209.