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/app.ts

@@ -9,10 +9,12 @@ import { FileActionsService } from './file-actions.service';
 import { FilePanel } from './file-panel/file-panel';
 import { RpcService } from './rpc.service';
 import { ToastService } from './toast.service';
+import { TransferBar } from './transfer-bar/transfer-bar';
+import { TransfersService } from './transfers.service';
 
 @Component({
   selector: 'app-root',
-  imports: [NgIcon, FilePanel, PromptDialog, ConfirmDialog, ChmodDialog],
+  imports: [NgIcon, FilePanel, PromptDialog, ConfirmDialog, ChmodDialog, TransferBar],
   templateUrl: './app.html',
 })
 export class App {
@@ -21,6 +23,7 @@ export class App {
   readonly toast = inject(ToastService);
   readonly dialogs = inject(DialogsService);
   readonly actions = inject(FileActionsService);
+  readonly transfers = inject(TransfersService);
 
   constructor() {
     document.title = BRANDING.appName;