feat(sp8): Claim drawer Download 837 button

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).

src/lib/api.ts

@@ -476,6 +476,42 @@ async function getClaimDetail(id: string): Promise<ClaimDetail> {
   return (await res.json()) as ClaimDetail;
 }
 
+/**
+ * Serialize a claim back to an X12 837P file via the backend's outbound
+ * serializer (SP8).
+ *
+ * Returns both the regenerated X12 text and the filename the backend
+ * suggested (from the `Content-Disposition: attachment; filename=...`
+ * header). Callers usually pipe both into `downloadTextFile` so the user
+ * gets a byte-faithful file named after the claim id.
+ *
+ * Throws `ApiError` on non-2xx — 404 (claim missing) and 422 (stored
+ * `raw_json` unparseable / serializer rejected the payload) are both
+ * reachable, and the caller may want to branch on `.status` to surface a
+ * useful message ("claim data is corrupted — open the batch file
+ * manually" vs. "this claim no longer exists").
+ */
+async function serializeClaim837(
+  id: string,
+): Promise<{ text: string; filename: string }> {
+  if (!isConfigured) throw notConfiguredError();
+  const res = await fetch(
+    joinUrl(`/api/claims/${encodeURIComponent(id)}/serialize-837`),
+  );
+  if (!res.ok) {
+    const detail = await readErrorBody(res);
+    throw new ApiError(res.status, detail || res.statusText);
+  }
+  const text = await res.text();
+  // Filename comes from Content-Disposition (set by the backend). Fall back
+  // to a sensible default if the header is missing or malformed so the UI
+  // always has something to hand to `downloadTextFile`.
+  const cd = res.headers.get("content-disposition") ?? "";
+  const match = /filename="?([^";]+)"?/i.exec(cd);
+  const filename = match?.[1] ?? `claim-${id}.x12`;
+  return { text, filename };
+}
+
 async function listRemittances<T = unknown>(
   params: ListRemittancesParams
 ): Promise<PaginatedResponse<T>> {
@@ -700,6 +736,7 @@ export const api = {
   getBatchDiff,
   listClaims,
   getClaimDetail,
+  serializeClaim837,
   listRemittances,
   getRemittance,
   listProviders,