Serial/parallel transfers setting

Transfers: Single | Multi in the options popover, with an 'At once' picker
(2/3/4/6/8) when Multi. Engine concurrency becomes an instance field applied
live: at boot from settings.json and on every saveSettings (raising the limit
starts queued jobs immediately). Verified headless: 3 slow jobs overlap at
concurrency 3, stay strictly serial at 1, clamp works.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/bun/transfers/engine.ts

@@ -5,8 +5,6 @@ import type { FileSystemProvider } from "../fs/provider";
 import type { SessionRegistry } from "../fs/registry";
 import { walkTree } from "./walk";
 
-// One at a time keeps remote servers happy; bump for parallel jobs.
-const TRANSFER_CONCURRENCY = 1;
 const PROGRESS_INTERVAL_MS = 100;
 
 interface Job {
@@ -41,9 +39,16 @@ export class TransferEngine {
 	private running = 0;
 	private counter = 0;
 	private lastEmit = new Map<string, number>();
+	// serial by default; the transfers setting raises it (jobs are independent)
+	private concurrency = 1;
 
 	constructor(private registry: SessionRegistry) {}
 
+	setConcurrency(n: number): void {
+		this.concurrency = Math.max(1, Math.min(8, Math.round(n) || 1));
+		this.pump(); // a raised limit starts queued jobs immediately
+	}
+
 	enqueue(params: {
 		srcSessionId: string;
 		items: { path: string; type: EntryType }[];
@@ -119,7 +124,7 @@ export class TransferEngine {
 	}
 
 	private pump(): void {
-		while (this.running < TRANSFER_CONCURRENCY && this.queue.length) {
+		while (this.running < this.concurrency && this.queue.length) {
 			const job = this.queue.shift()!;
 			this.running++;
 			this.run(job)