feat(batch-diff): side-by-side claim diff between two batches

src/hooks/useBatchDiff.ts

@@ -0,0 +1,35 @@
+import { useQuery } from "@tanstack/react-query";
+import { api, ApiError } from "@/lib/api";
+import type { BatchDiff } from "@/types";
+
+/**
+ * React Query binding for the side-by-side batch diff.
+ *
+ * Query key: ``["batch-diff", a, b]`` so picking different batches
+ * automatically produces a separate cache entry. ``enabled`` short-
+ * circuits when either id is missing — the page disables the Run
+ * button until both pickers are filled in, but we double-check here
+ * so a half-picked URL can't fire an empty query.
+ *
+ * 404s from the backend are surfaced as ``ApiError(404)`` — the page
+ * branches on ``error.status === 404`` to render the "one of these
+ * batches was deleted out from under you" state instead of the
+ * generic error toast. Matches the same convention used by the
+ * claim detail drawer.
+ */
+export function useBatchDiff(a: string | null, b: string | null) {
+  return useQuery<BatchDiff, ApiError>({
+    queryKey: ["batch-diff", a, b],
+    queryFn: () => api.getBatchDiff(a as string, b as string),
+    enabled:
+      api.isConfigured &&
+      typeof a === "string" &&
+      a.length > 0 &&
+      typeof b === "string" &&
+      b.length > 0,
+    // Refetch on window focus would be nice in production; for the
+    // diff view it's pointless because the underlying batches don't
+    // change shape without a fresh ingest.
+    refetchOnWindowFocus: false,
+  });
+}