fix(onboarding): use POST for gateway health check in wizard (#334)

The test connection button in the onboarding wizard was using GET to
call /api/gateways/health, which only exports POST — resulting in a
405 and "Gateway unreachable" shown to the user.

Fixes the HTTP method to POST so the health probe works correctly.

Co-authored-by: Firefloc <firefloc@MacBook-Pro-de-Firefloc.local>

src/components/onboarding/onboarding-wizard.tsx

@@ -529,7 +529,7 @@ function StepGatewayLink({ isGateway, registration, onNext, onBack }: {
   const testConnection = async () => {
     setTesting(true)
     try {
-      const res = await fetch('/api/gateways/health')
+      const res = await fetch('/api/gateways/health', { method: 'POST' })
       setHealthOk(res.ok)
     } catch {
       setHealthOk(false)