Implemented phase 5 for the POC prototype in the Website service.

Services/Website/Resources/Static/js/passkey.js

@@ -1,4 +1,4 @@
-// CCN — passkey sign-in. Without it, or without WebAuthn, the sign-in screen offers the demo sign-in alone.
+// CCN — passkeys: signing in or joining, and adding one from the account screen. Without it, or without WebAuthn, the demo sign-in remains.
 
 const form = document.querySelector("[data-passkey-form]");
 const supported = window.PublicKeyCredential
@@ -30,7 +30,10 @@ if (form && supported) {
     status.textContent = "";
 
     try {
-      const { mode, options } = await post("/sign-in/passkey/options", { email: form.elements.email.value });
+      // On the account screen the button adds a passkey to the account signed in; on the sign-in screen it signs in or joins.
+      const adding = button.dataset.passkey === "add";
+      const start = adding ? { add: true } : { email: form.querySelector("[name=email]").value };
+      const { mode, options } = await post("/sign-in/passkey/options", start);
       let credential;
 
       if (mode === "create") {
@@ -47,7 +50,7 @@ if (form && supported) {
 
       const { location } = await post("/sign-in/passkey/finish", {
         credential: credential.toJSON(),
-        return: form.elements.return.value,
+        return: form.querySelector("[name=return]")?.value,
       });
 
       window.location.assign(location);