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/shouts/route.ts

@@ -6,7 +6,6 @@ import { db, RadioShouts, User } from "@/lib/db";
 // Latest 50 radio shouts with their author's username/look resolved. Mirrors the
 // query behind the public /radio/shouts page (radio_shouts ordered by created_at
 // desc, then joined to users by user_id).
-export const dynamic = "force-dynamic";
 
 // Max shout length (radio_shouts.message is TEXT; cap to keep posts sane).
 const MAX_MESSAGE_LENGTH = 255;