refactor(inspect-ui): rename Step.action to Step.next_action

Aligns the SPA type and consumers with the trace schema rename. The
StepSummary.action_kind/action_label labels stay unchanged since they
are derived labels, not the raw next-action.

inspect-ui/src/routes/RunDetail.tsx

@@ -127,7 +127,7 @@ export default function RunDetail() {
     {
       id: "screenshot",
       label: "Screenshot",
-      content: <Screenshot src={beforeScreenshot} action={currentStep?.action} />,
+      content: <Screenshot src={beforeScreenshot} action={currentStep?.next_action} />,
     },
     {
       id: "snapshots",