Onboarding: set https cookie + auto redirect

pikit-web/onboarding/index.html

@@ -95,6 +95,7 @@
           try {
             await fetch(`${target}/api/status`, { mode: "no-cors", cache: "no-store" });
             log("HTTPS reachable, redirecting");
+            document.cookie = "pikit_https_ok=1; Path=/; Max-Age=2592000; SameSite=Lax";
             window.location = target;
           } catch (e) {
             log("HTTPS probe failed; staying on onboarding page");