feat(collect): remove power-on/off, add skip-hung for Redfish collection

Remove power-on and power-off functionality from the Redfish collector;
keep host power-state detection and show a warning in the UI when the
host is powered off before collection starts.

Add a "Пропустить зависшие" (skip hung) button that lets the user abort
stuck Redfish collection phases without losing already-collected data.
Introduces a two-level context model in Collect(): the outer job context
covers the full lifecycle including replay; an inner collectCtx covers
snapshot, prefetch, and plan-B phases only. Closing the skipCh cancels
collectCtx immediately — aborts all in-flight HTTP requests and exits
plan-B loops — then replay runs on whatever rawTree was collected.

Signal path: UI → POST /api/collect/{id}/skip → JobManager.SkipJob()
→ close(skipCh) → goroutine in Collect() → cancelCollect().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/server/collect_handlers_test.go

@@ -24,6 +24,7 @@ func newCollectTestServer() (*Server, *httptest.Server) {
 	mux.HandleFunc("POST /api/collect", s.handleCollectStart)
 	mux.HandleFunc("GET /api/collect/{id}", s.handleCollectStatus)
 	mux.HandleFunc("POST /api/collect/{id}/cancel", s.handleCollectCancel)
+	mux.HandleFunc("POST /api/collect/{id}/skip", s.handleCollectSkip)
 	return s, httptest.NewServer(mux)
 }
 
@@ -65,9 +66,6 @@ func TestCollectProbe(t *testing.T) {
 	if payload.HostPowerState != "Off" {
 		t.Fatalf("expected host power state Off, got %q", payload.HostPowerState)
 	}
-	if !payload.PowerControlAvailable {
-		t.Fatalf("expected power control to be available")
-	}
 }
 
 func TestCollectLifecycleToTerminal(t *testing.T) {