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_test_helpers_test.go

@@ -26,12 +26,11 @@ func (c *mockConnector) Probe(ctx context.Context, req collector.Request) (*coll
 		hostPoweredOn = false
 	}
 	return &collector.ProbeResult{
-		Reachable:             true,
-		Protocol:              c.protocol,
-		HostPowerState:        map[bool]string{true: "On", false: "Off"}[hostPoweredOn],
-		HostPoweredOn:         hostPoweredOn,
-		PowerControlAvailable: true,
-		SystemPath:            "/redfish/v1/Systems/1",
+		Reachable:      true,
+		Protocol:       c.protocol,
+		HostPowerState: map[bool]string{true: "On", false: "Off"}[hostPoweredOn],
+		HostPoweredOn:  hostPoweredOn,
+		SystemPath:     "/redfish/v1/Systems/1",
 	}, nil
 }