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/online/count/route.ts

@@ -8,8 +8,6 @@ import { apiJson } from "@/lib/api";
 import { cached } from "@/lib/cache";
 import { db, User } from "@/lib/db";
 
-export const dynamic = "force-dynamic";
-
 export async function GET(_req: Request) {
 	try {
 		const result = await cached("online_count", 10_000, async () => {

src/app/api/online/route.ts

@@ -8,8 +8,6 @@ import { apiJson } from "@/lib/api";
 import { cached } from "@/lib/cache";
 import { db, User } from "@/lib/db";
 
-export const dynamic = "force-dynamic";
-
 export async function GET(_req: Request) {
 	try {
 		const users = await cached("online_users", 10_000, async () =>