PO History had no way to narrow by accounting code. Add an "Accounting
Code" filter (the shared type-to-search combobox) alongside Cost Centre,
backed by the PO-level account already included in the query.
- history/page.tsx: read `accountId` searchParam, fetch selectable leaf
accounting codes (active, no children) via buildAccountGroups, apply
`where.accountId`, thread the param into pagination + export links, and
surface an Accounting Code column for context.
- history-filters.tsx: new SearchableSelect control wired into
buildParams/apply/clear/hasFilters like the Cost Centre select.
- api/reports/export: apply the same `accountId` filter so CSV/PDF export
respects the on-screen filter.
- tests/staging: verify picking a code drives an `accountId` query param.
Fixes#121
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
3e8f5fb0c7