perf(import): adjustable concurrency and skip redundant SWF downloads

Raise batch concurrency (furni 3->12, clone 10->12) with a Speed control next to Translate. Skip the SWF download when a .nitro bundle already exists on disk (color variants share the base nitro), and stop flagging that as a failed download.

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

@@ -269,6 +269,8 @@ export function StudioClient({
 		null,
 	);
 	const batchAbortRef = useRef<AbortController | null>(null);
+	// How many items the batch endpoints process in parallel.
+	const [concurrency, setConcurrency] = useState(4);
 
 	// Nitro editor
 	const [editingClassname, setEditingClassname] = useState<string | null>(null);
@@ -834,7 +836,7 @@ export function StudioClient({
 						revision: i.revision,
 						category: i.category,
 					})),
-					concurrency: 3,
+					concurrency,
 					translate,
 					langs:
 						translate && translateLangs.length > 0 ? translateLangs : undefined,
@@ -979,7 +981,7 @@ export function StudioClient({
 				body: JSON.stringify({
 					sourceId: activeSource,
 					classnames: failed,
-					concurrency: 10,
+					concurrency,
 					final: true,
 				}),
 				signal: abort.signal,
@@ -1561,6 +1563,33 @@ export function StudioClient({
 				</summary>
 				<div className="flex max-h-48 flex-wrap items-center gap-3 overflow-auto py-3">
 					{" "}
+					{/* ── Import speed (parallel items per batch) ─────────── */}
+					<div className="flex items-center gap-2">
+						<span className="text-xs font-medium text-[var(--admin-text-muted)]">
+							Speed
+						</span>
+						<Select
+							value={String(concurrency)}
+							onValueChange={(value) =>
+								setConcurrency(Math.max(1, Number(value)))
+							}
+						>
+							<SelectTrigger
+								aria-label="Import speed"
+								className="h-8 w-24 text-xs"
+							>
+								<SelectValue>{`${concurrency} parallel`}</SelectValue>
+							</SelectTrigger>
+							<SelectContent>
+								<SelectItem value="1">1 (gentle)</SelectItem>
+								<SelectItem value="2">2</SelectItem>
+								<SelectItem value="4">4 (recommended)</SelectItem>
+								<SelectItem value="6">6</SelectItem>
+								<SelectItem value="8">8</SelectItem>
+								<SelectItem value="12">12 (turbo)</SelectItem>
+							</SelectContent>
+						</Select>
+					</div>
 					{/* ── Translate control (per-import) ─────────────── */}
 					<div className="flex items-center gap-2">
 						<div className="flex h-8 items-center gap-1.5 rounded-md border border-[var(--admin-border)] px-2.5">