feat(auth): wire AuthProvider + RequireAuth into app shell

src/auth/api.ts

@@ -88,6 +88,54 @@ export async function authedFetch<T = unknown>(
   return (await res.json()) as T;
 }
 
+/**
+ * Like `authedFetch` but returns the response body as text instead of
+ * parsed JSON. Used by `serializeClaim837` (the SP8 endpoint returns
+ * `text/x12`, not JSON). Same 401-redirect + error-shape behavior as
+ * the JSON variant.
+ */
+export async function authedFetchText(path: string, init?: RequestInit): Promise<string> {
+  const res = await fetch(joinUrl(path), {
+    credentials: "include",
+    ...init,
+  });
+  if (res.status === 401 && !path.startsWith("/api/auth/")) {
+    redirectToLogin();
+    throw new ApiError(401, "session_expired");
+  }
+  if (!res.ok) {
+    let body: any = null;
+    try {
+      body = await res.json();
+    } catch {
+      /* no body */
+    }
+    throw new ApiError(
+      res.status,
+      body?.error ?? "error",
+      body?.detail ?? res.statusText
+    );
+  }
+  return res.text();
+}
+
+/**
+ * Like `authedFetch` but returns the raw `Response` so the caller can
+ * stream the body (NDJSON). Used by `parse837` / `parse835`. 401 still
+ * redirects; the caller is responsible for reading the body.
+ */
+export async function authedFetchResponse(path: string, init?: RequestInit): Promise<Response> {
+  const res = await fetch(joinUrl(path), {
+    credentials: "include",
+    ...init,
+  });
+  if (res.status === 401 && !path.startsWith("/api/auth/")) {
+    redirectToLogin();
+    throw new ApiError(401, "session_expired");
+  }
+  return res;
+}
+
 // ---------------------------------------------------------------------------
 // Auth-specific endpoints. `login` and `logout` use raw `fetch` because
 // they bypass the 401-redirect behavior on purpose — a 401 from