fix: use 0-based step order when creating funnels

Backend expects sequential order starting from 0 (0, 1, 2, ...),
but the frontend was sending 1-based order (1, 2, 3, ...).

app/sites/[id]/funnels/new/page.tsx

@@ -72,7 +72,7 @@ export default function CreateFunnelPage() {
       setSaving(true)
       const funnelSteps = steps.map((s, i) => ({
         ...s,
-        order: i + 1
+        order: i
       }))
 
       await createFunnel(siteId, {