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/transfer-bar/transfer-bar.html

@@ -0,0 +1,38 @@
+@if (transfers.hasJobs()) {
+  <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">
+        <ng-icon [name]="stateIcon(job)" size="0.95rem" [class]="stateClass(job)" />
+        <span class="w-40 shrink-0 truncate font-medium" [title]="job.srcPath">{{ job.name }}</span>
+        <div class="min-w-0 flex-1">
+          <div class="truncate text-[11px] text-muted-foreground">{{ progressLabel(job) }}</div>
+          @if (isActive(job)) {
+            <div class="mt-0.5 h-1 overflow-hidden rounded-full bg-muted">
+              <div
+                class="h-full rounded-full bg-primary transition-[width] duration-100"
+                [style.width.%]="jobPercent(job)"
+              ></div>
+            </div>
+          }
+        </div>
+        @if (isActive(job)) {
+          <button
+            (click)="transfers.cancel(job.id)"
+            title="Cancel"
+            class="shrink-0 rounded p-1 text-muted-foreground hover:bg-accent hover:text-destructive"
+          >
+            <ng-icon name="mynaX" size="0.9rem" />
+          </button>
+        } @else {
+          <button
+            (click)="transfers.dismiss(job.id)"
+            title="Dismiss"
+            class="shrink-0 rounded p-1 text-muted-foreground hover:bg-accent hover:text-foreground"
+          >
+            <ng-icon name="mynaX" size="0.9rem" />
+          </button>
+        }
+      </div>
+    }
+  </div>
+}