Finished-transfers setting + transfer-list header with Clear finished

- Options popover: 'Finished transfers' Auto-clear | Keep (default auto-clear,
  the previous behavior); persisted like the rest
- Transfer list header: 'N active · M finished' + Clear finished button
  (dismisses completed/failed/cancelled only, never running/queued)
- Failed rows now name the file that failed alongside the error message

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

src/shared/types.ts

@@ -50,6 +50,8 @@ export interface AppSettings {
 	transferMode: "single" | "multi";
 	/** How many jobs run at once when transferMode is "multi". */
 	transferConcurrency: number;
+	/** Remove completed transfers from the list automatically. */
+	autoClearCompleted: boolean;
 }
 
 export const DEFAULT_SETTINGS: AppSettings = {
@@ -58,6 +60,7 @@ export const DEFAULT_SETTINGS: AppSettings = {
 	textSize: "default",
 	transferMode: "single",
 	transferConcurrency: 3,
+	autoClearCompleted: true,
 };
 
 export function effectiveConcurrency(settings: AppSettings): number {