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/lib/services/import/core/sse-batch.ts

@@ -33,7 +33,7 @@ export interface RunSseBatchOptions<T> {
  */
 export function runSseBatch<T>(opts: RunSseBatchOptions<T>): Response {
 	const { items, labelOf, worker, signal, flush } = opts;
-	const concurrency = Math.min(Math.max(opts.concurrency || 3, 1), 10);
+	const concurrency = Math.min(Math.max(opts.concurrency || 3, 1), 12);
 	const encoder = new TextEncoder();
 	const ac = new AbortController();