The PO Activity timeline rendered every partial payment as the generic
"Partial payment confirmed". markPaid() already persists the instalment
amount on the PARTIAL_PAYMENT_CONFIRMED action's metadata
(metadata.paymentAmount), so surface it: the row now reads
"Partial payment of <amount> confirmed" using the PO's own currency.
Falls back to the plain label when paymentAmount is missing or
non-numeric (older audit rows) so historical POs never render NaN.
Extracted ACTION_LABELS + the new actionLabel() helper into
lib/po-activity.ts so the label logic is unit-testable without pulling
the server-only PoDetail component (and its storage/auth imports) into
jsdom.
Fixes#140
