All PO attachments are stored as PODocument rows whose lifecycle stage
(submission vs delivery) is encoded in the storageKey prefix. The PO
details screen previously listed them in a single flat "Attachments"
block, giving no indication of which were submission documents (invoice,
quotation) versus delivery receipts.
Add lib/attachments.ts to derive a user-facing group from the storageKey
prefix (submission / payment / delivery / other) and render each
non-empty group as a labelled subsection on the PO details screen, in
lifecycle order. Unknown prefixes fall back to an "Other" group so
nothing is ever hidden.
Fixes#10
