P3: transfer engine with queue, progress, cancel + transfer bar UI

- TransferEngine: FIFO queue, provider-generic stream pipeline (covers all four
  panel combos), throttled transferUpdate push, cancel via stream destroy,
  self-copy guards, failSession hook for dead SSH sessions
- TransfersService + TransferBar: per-job rows, progress bars, cancel/dismiss,
  auto-dismiss completed, dst panel refresh
- F5 + Copy button wired to active-panel selection
- verified: 30MB nested tree copy (UTF-8 names, empty dirs), self-copy rejection,
  mid-transfer cancel leaving partial file

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

ui/src/app/file-panel/file-panel.ts

@@ -13,6 +13,7 @@ import type { FileEntry } from '@shared/types';
 import { FileActionsService } from '../file-actions.service';
 import { formatDate, formatSize } from '../format';
 import { PanelsService, type Side, type SortKey } from '../panels.service';
+import { TransfersService } from '../transfers.service';
 
 @Component({
   selector: 'app-file-panel',
@@ -25,6 +26,7 @@ export class FilePanel {
 
   readonly panels = inject(PanelsService);
   readonly actions = inject(FileActionsService);
+  readonly transfers = inject(TransfersService);
   readonly panel = computed(() => this.panels.get(this.side()));
   readonly isActive = computed(() => this.panels.activeSide() === this.side());
 
@@ -144,6 +146,9 @@ export class FilePanel {
       case 'F2':
         this.actions.rename();
         break;
+      case 'F5':
+        void this.transfers.copyToOtherPanel();
+        break;
       case 'F7':
         this.actions.mkdir();
         break;