perf: offload FurnitureData translation build to a worker thread

The translation build's CPU-heavy work (deep clone of the 100k+ entry
master, the two full scans and JSON.stringify per language) now runs in a
worker thread, so "Alles vertalen" no longer blocks the main event loop.
Network/DB parts (official Habbo fetch, LibreTranslate, file writes) stay
on the main thread. The pure helpers were extracted to furni-data-i18n-core
(no server-only deps) so the worker can import them. Falls back to the same
helpers on the main thread when no worker is available or in tests.

src/lib/services/translation-worker.ts

@@ -0,0 +1,76 @@
+import { parentPort } from "node:worker_threads";
+import type { HabboGamedataHotel } from "@/lib/habbo-gamedata-hotel";
+import {
+	type FurniDataMaster,
+	finalizeLangFile,
+	type PreparedLang,
+	prepareLangFile,
+	type Translation,
+} from "./furni-data-i18n-core";
+
+interface InitMsg {
+	id: number;
+	type: "init";
+	masterJson: string;
+	hotelMaps: Record<string, Map<string, Translation>>;
+}
+interface PrepareMsg {
+	id: number;
+	type: "prepare";
+	lang: string;
+	hotel: string;
+}
+interface FinalizeMsg {
+	id: number;
+	type: "finalize";
+	lang: string;
+	translations: Map<string, string>;
+}
+type Msg = InitMsg | PrepareMsg | FinalizeMsg;
+
+let master: FurniDataMaster | null = null;
+let hotelMaps: Record<string, Map<string, Translation>> = {};
+const prepared = new Map<string, PreparedLang>();
+
+if (parentPort) {
+	parentPort.on("message", (msg: Msg) => {
+		try {
+			if (msg.type === "init") {
+				master = JSON.parse(msg.masterJson);
+				hotelMaps = msg.hotelMaps ?? {};
+				parentPort!.postMessage({ id: msg.id, ok: true });
+				return;
+			}
+			if (msg.type === "prepare") {
+				if (!master) throw new Error("translation worker not initialized");
+				const officialMap =
+					hotelMaps[msg.hotel] ?? new Map<string, Translation>();
+				const r = prepareLangFile(
+					master,
+					msg.lang,
+					msg.hotel as HabboGamedataHotel,
+					officialMap,
+				);
+				prepared.set(msg.lang, r);
+				parentPort!.postMessage({
+					id: msg.id,
+					ok: true,
+					pendingTexts: r.pendingTexts,
+				});
+				return;
+			}
+			// finalize
+			const r = prepared.get(msg.lang);
+			if (!r) throw new Error(`no prepared data for ${msg.lang}`);
+			const res = finalizeLangFile(r.cloned, r.pendingRefs, msg.translations);
+			prepared.delete(msg.lang);
+			parentPort!.postMessage({ id: msg.id, ok: true, ...res });
+		} catch (err) {
+			parentPort!.postMessage({
+				id: msg.id,
+				ok: false,
+				error: (err as Error).message,
+			});
+		}
+	});
+}