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>

ui/src/app/tab-bar/tab-bar.ts

@@ -187,6 +187,30 @@ const CONCURRENCY_CHOICES = [2, 3, 4, 6, 8] as const;
               </div>
             </div>
           }
+
+          <div class="mt-2.5 flex items-center justify-between gap-2">
+            <span class="text-xs text-muted-foreground">Finished transfers</span>
+            <div class="flex rounded-md border border-border p-0.5">
+              <button
+                (click)="settings.setAutoClearCompleted(true)"
+                class="rounded px-2 py-0.5 text-[11px]"
+                [class.bg-accent]="settings.autoClearCompleted()"
+                [class.text-foreground]="settings.autoClearCompleted()"
+                [class.text-muted-foreground]="!settings.autoClearCompleted()"
+              >
+                Auto-clear
+              </button>
+              <button
+                (click)="settings.setAutoClearCompleted(false)"
+                class="rounded px-2 py-0.5 text-[11px]"
+                [class.bg-accent]="!settings.autoClearCompleted()"
+                [class.text-foreground]="!settings.autoClearCompleted()"
+                [class.text-muted-foreground]="settings.autoClearCompleted()"
+              >
+                Keep
+              </button>
+            </div>
+          </div>
         </div>
       }
     </div>