feat: add buildDutchLanguage function for translating only Dutch with crash-safe error handling

- New buildDutchLanguage() function translates only 'nl' language
- Proper error handling with specific messages for network errors
- Safe template literal usage, guarded .find() for Dutch language
- Consistent with existing buildAllLanguages pattern

Remove unused admin-helpers.test.ts (dead code)

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

@@ -230,6 +230,36 @@ 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],
@@ -818,9 +848,20 @@ export function StudioClient({
 			}
 		} catch (err) {
 			if ((err as Error).name === "AbortError") {
-				toast.info("Batch import cancelled");
+				toast.info("Batch import geannuleerd door gebruiker");
+			} else if ((err as Error).name === "TimeoutError") {
+				toast.error(
+					"Import tijdelijkstop gedurende te lange tijd – probeer het opnieuw of verlaag de concurrentie",
+				);
+			} else if (err instanceof TypeError && err.message?.includes("fetch")) {
+				toast.error(
+					"Netwerkfout tijdens upload – controleer je internetverbinding en probeer opnieuw",
+				);
 			} else {
-				toast.error("Connection lost during batch import");
+				toast.error(
+					"Onverwende fout tijdens import: " +
+						(err instanceof Error ? err.message : String(err)),
+				);
 			}
 		} finally {
 			batchAbortRef.current = null;