feat: speed up admin clone import and enable Cache Components

- Clone import: defer FurnitureData.json writes and append all entries in a
  single batched write instead of one read-modify-write per item, removing
  the main serialization bottleneck for large batches.
- Clone import: raise SSE batch concurrency cap from 5 to 10 and bump the
  clone client/route default from 2 to 6.
- Add a flush hook to runSseBatch so callers can batch deferred work before
  batch_complete is emitted, and surface flush errors as an error event.
- Enable Next.js Cache Components (instant: false opt-out) and silence the
  related build warnings in next.config.ts.
- Switch isomorphic-dompurify to dompurify and refresh dependencies.

src/lib/sse-client.ts

@@ -88,6 +88,13 @@ export async function runSseImport(
 					onFailed?.(classname, String(evt.error ?? ""));
 				}
 			}
+			if (evt.type === "error") {
+				toast.error(
+					typeof evt.message === "string"
+						? evt.message
+						: "Import finished with errors",
+				);
+			}
 			if (evt.type === "batch_complete") {
 				succeeded = Number(evt.succeeded ?? 0);
 				failed = Number(evt.failed ?? 0);