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/shared/types.ts

@@ -46,14 +46,25 @@ export interface AppSettings {
 	speedUnit: "MBps" | "Mbps";
 	/** Font size of the path bar and file list. */
 	textSize: "compact" | "default" | "large";
+	/** Run transfer jobs one at a time, or several in parallel. */
+	transferMode: "single" | "multi";
+	/** How many jobs run at once when transferMode is "multi". */
+	transferConcurrency: number;
 }
 
 export const DEFAULT_SETTINGS: AppSettings = {
 	theme: "system",
 	speedUnit: "MBps",
 	textSize: "default",
+	transferMode: "single",
+	transferConcurrency: 3,
 };
 
+export function effectiveConcurrency(settings: AppSettings): number {
+	if (settings.transferMode === "single") return 1;
+	return Math.max(1, Math.min(8, Math.round(settings.transferConcurrency) || 1));
+}
+
 export type TransferState =
 	| "queued"
 	| "scanning"