Three pieces:
- src/lib/download.ts: generic downloadTextFile(filename, mime, text)
helper. Mirrors csv.ts:downloadCsv but takes an explicit MIME type and
drops the BOM prepend (which would corrupt the ISA segment).
- src/lib/api.ts: serializeClaim837(id) → {text, filename}. Fetches
GET /api/claims/{id}/serialize-837, pulls the suggested filename from
Content-Disposition (falls back to claim-{id}.x12 if the header is
missing). Throws ApiError on non-2xx so callers can branch on .status.
- ClaimDrawerHeader: Download icon button between the amount and the
close button. Click → api.serializeClaim837 → downloadTextFile.
Disabled + 'Downloading 837 file' aria-label while the fetch is in
flight so the click feels responsive. Optional onError prop surfaces
fetch failures; defaults to a no-op so existing callers stay clean.
Tests: 3 download.test.ts, 3 api.test.ts, 2 header.test.ts (happy
path + error path). Frontend: 350 passing (+8 from 342).
Tyler