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.html

@@ -20,8 +20,17 @@
       <app-file-panel side="right" />
     </main>
 
+    <app-transfer-bar />
+
     <!-- function bar: acts on the active panel -->
     <footer class="flex items-center gap-1 border-t border-border px-2 py-1">
+      <button
+        (click)="transfers.copyToOtherPanel()"
+        title="Copy to other panel (F5)"
+        class="flex items-center gap-1.5 rounded-md px-2 py-1 text-xs text-muted-foreground hover:bg-accent hover:text-foreground"
+      >
+        <ng-icon name="mynaArrowLeftRight" size="0.9rem" /> Copy
+      </button>
       <button
         (click)="actions.rename()"
         title="Rename (F2)"