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/index.ts

@@ -1,5 +1,6 @@
 import { BrowserWindow, Updater } from "electrobun/bun";
 import { BRANDING } from "../shared/branding";
+import { effectiveConcurrency } from "../shared/types";
 import { SessionRegistry } from "./fs/registry";
 import { createAppRPC } from "./rpc-handlers";
 import { ConnectionStore } from "./store/connections";
@@ -58,5 +59,6 @@ new BrowserWindow({
 
 engine.onUpdate = (snapshot) => rpc.send.transferUpdate(snapshot);
 events.sessionClosed = (info) => rpc.send.sessionClosed(info);
+void settings.load().then((s) => engine.setConcurrency(effectiveConcurrency(s)));
 
 console.log(`${BRANDING.appName} started (${url})`);

src/bun/rpc-handlers.ts

@@ -1,6 +1,6 @@
 import { BrowserView } from "electrobun/bun";
 import type { AppRPC } from "../shared/rpc";
-import type { Result } from "../shared/types";
+import { effectiveConcurrency, type Result } from "../shared/types";
 import type { SessionRegistry } from "./fs/registry";
 import { SftpProvider } from "./fs/sftp";
 import type { ConnectionStore } from "./store/connections";
@@ -144,6 +144,7 @@ export function createAppRPC({ registry, engine, store, settings, events }: AppD
 				saveSettings: ({ settings: next }) =>
 					wrap(async () => {
 						await settings.save(next);
+						engine.setConcurrency(effectiveConcurrency(next));
 						return {};
 					}),
 			},

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)