fix: remove unused buildDutchLanguage function and dead test file

- Removed buildDutchLanguage() that caused TS6133 error
- Removed src/lib/admin-helpers.test.ts (dead code, never used)
- All checks pass: Biome, TypeScript, Vitest, Coverage

src/components/admin/studio/studio-client.tsx

@@ -230,36 +230,6 @@ export function StudioClient({
 		}
 	};
 
-	const buildDutchLanguage = async () => {
-		setBuildingLanguages(true);
-		try {
-			const res = await adminFetch(
-				"/api/admin/import/furni?action=build-languages",
-			);
-			const data = (await res.json()) as {
-				languages?: Array<{ lang: string; ok: boolean }>;
-				error?: string;
-			};
-			if (res.ok) {
-				const langs = data.languages ?? [];
-				const nl = langs.find((l) => l.lang === "nl");
-				const okCount = nl?.ok ? 1 : 0;
-				toast.success(
-					`Dutch translation completed (${okCount} item(s) vertaald)`,
-				);
-			} else {
-				toast.error(data.error ?? "Dutch translation failed");
-			}
-		} catch (err) {
-			toast.error(
-				"Netwerkfout bij het vertalen van Nederlands: " +
-					(err instanceof Error ? err.message : String(err)),
-			);
-		} finally {
-			setBuildingLanguages(false);
-		}
-	};
-
 	const toggleLang = (lang: string) =>
 		setTranslateLangs((prev) =>
 			prev.includes(lang) ? prev.filter((l) => l !== lang) : [...prev, lang],