Change WebAuthn create challenge to 16 random bytes (#2561)

According to the WebAuthn spec the challenge _should_ be at
least 16 bytes. So far it was 9. Apparently, KeePassXC verifies
the challenge length and refuses to sign shorter values.

This changes the length to 16 bytes which should address the problem.

Closes #2560.

src/frontend/src/utils/iiConnection.ts

@@ -762,7 +762,7 @@ export const creationOptions = (
             type: "public-key",
           }
     ),
-    challenge: Uint8Array.from("<ic0.app>", (c) => c.charCodeAt(0)),
+    challenge: window.crypto.getRandomValues(new Uint8Array(16)),
     pubKeyCredParams: [
       {
         type: "public-key",