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/download.test.ts

@@ -0,0 +1,97 @@
+// @vitest-environment happy-dom
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { downloadTextFile } from "./download";
+
+// Capture the real appendChild before any spy replaces it, so the
+// happy-dom implementation is preserved when the appendChild spy calls
+// through (otherwise removeChild inside downloadTextFile can't find the
+// node we appended).
+const originalAppendChild = document.body.appendChild.bind(document.body);
+
+/**
+ * `downloadTextFile` drives a hidden `<a download>` and clicks it. The
+ * observable side effects we care about:
+ *
+ *   1. An `<a>` element with the right `href` (object URL) and `download`
+ *      (filename) is appended to `document.body`.
+ *   2. The element is removed from the DOM (cleanup).
+ *   3. The object URL is revoked (memory hygiene).
+ *
+ * We don't try to assert on the browser's actual download behavior — that's
+ * not observable from the test environment. We assert on the contract the
+ * implementation honors.
+ */
+describe("downloadTextFile", () => {
+  let createSpy: ReturnType<typeof vi.spyOn>;
+  let revokeSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    createSpy = vi
+      .spyOn(URL, "createObjectURL")
+      .mockReturnValue("blob:test-url");
+    revokeSpy = vi.spyOn(URL, "revokeObjectURL");
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("test_appends_anchor_with_object_url_and_filename_then_cleans_up", async () => {
+    // Spy on appendChild to capture the anchor that's about to be appended
+    // (downloadTextFile removes it again synchronously, so lastElementChild
+    // is back to null by the time the call returns).
+    const captured: HTMLAnchorElement[] = [];
+    const appendSpy = vi
+      .spyOn(document.body, "appendChild")
+      .mockImplementation((node) => {
+        if (node instanceof HTMLAnchorElement) captured.push(node);
+        // Preserve the real append side-effect so removeChild later in
+        // downloadTextFile can find the node.
+        return originalAppendChild.call(document.body, node);
+      });
+
+    downloadTextFile("claim-CLM-1.x12", "text/x12", "ISA*00*~");
+
+    expect(captured).toHaveLength(1);
+    const a = captured[0];
+    expect(a.tagName).toBe("A");
+    expect(a.getAttribute("download")).toBe("claim-CLM-1.x12");
+    expect(a.getAttribute("href")).toBe("blob:test-url");
+    expect(a.style.display).toBe("none");
+
+    // removeChild ran synchronously inside downloadTextFile — the anchor
+    // is no longer a child of document.body.
+    expect(document.body.contains(a)).toBe(false);
+
+    expect(appendSpy).toHaveBeenCalledTimes(1);
+    expect(createSpy).toHaveBeenCalledTimes(1);
+    // The revoke is scheduled via queueMicrotask — flush the microtask
+    // queue so the assertion sees it.
+    await Promise.resolve();
+    expect(revokeSpy).toHaveBeenCalledWith("blob:test-url");
+  });
+
+  it("test_blob_carries_supplied_mime_type_and_text_bytes", () => {
+    downloadTextFile("a.txt", "text/plain", "hello world");
+
+    expect(createSpy).toHaveBeenCalledTimes(1);
+    const blob = createSpy.mock.calls[0][0] as Blob;
+    expect(blob.type).toBe("text/plain;charset=utf-8");
+    expect(blob.size).toBe("hello world".length);
+  });
+
+  it("test_noop_when_document_undefined", () => {
+    // SSR-style guard: with no `document`, the function returns silently
+    // instead of throwing.
+    const originalDocument = globalThis.document;
+    // @ts-expect-error - intentionally strip document for this case
+    delete (globalThis as { document?: Document }).document;
+    try {
+      expect(() =>
+        downloadTextFile("a.txt", "text/plain", "x"),
+      ).not.toThrow();
+    } finally {
+      (globalThis as { document?: Document }).document = originalDocument;
+    }
+  });
+});