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/app/api/radio/now-playing/route.ts

@@ -4,7 +4,6 @@ import { siteSettings } from "@/lib/services/site-settings";
 // Proxy for the configured radio "now playing" provider. The provider URL is
 // stored in radio_now_playing_api_url; we fetch it server-side (never exposing
 // the URL or any provider key to the browser) with a short, hard timeout.
-export const dynamic = "force-dynamic";
 
 const FETCH_TIMEOUT_MS = 4000;