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/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 {};
 					}),
 			},