feat(journeys): add skeleton and error boundary

app/sites/[id]/journeys/error.tsx

@@ -0,0 +1,13 @@
+'use client'
+
+import ErrorDisplay from '@/components/ErrorDisplay'
+
+export default function JourneysError({ reset }: { error: Error; reset: () => void }) {
+  return (
+    <ErrorDisplay
+      title="Journeys failed to load"
+      message="We couldn't load the journey data. This might be a temporary issue — try again."
+      onRetry={reset}
+    />
+  )
+}