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/transfer-bar/transfer-bar.html

@@ -1,4 +1,16 @@
 @if (transfers.expanded() && transfers.hasJobs()) {
+  <div class="flex items-center justify-between border-t border-border px-3 py-1 text-[11px] text-muted-foreground">
+    <span>
+      {{ transfers.activeJobs().length }} active · {{ transfers.finishedJobs().length }} finished
+    </span>
+    <button
+      (click)="transfers.clearFinished()"
+      [disabled]="!transfers.finishedJobs().length"
+      class="flex items-center gap-1 rounded px-1.5 py-0.5 hover:bg-accent hover:text-foreground disabled:opacity-40 disabled:hover:bg-transparent"
+    >
+      <ng-icon name="mynaTrash" size="0.75rem" /> Clear finished
+    </button>
+  </div>
   <div class="max-h-40 overflow-y-auto border-t border-border">
     @for (job of transfers.jobs(); track job.id) {
       <div class="flex items-center gap-2 px-3 py-1.5 text-xs">