Auto-suggest name for new Passkeys

- Use plain client data json everywhere
- Expose AAGUID and backed_up on Credentials
- Make attestation verifiers configurable
- Auto-suggest names for passkeys and show icons

app/javascript/helpers/base64url_helpers.js

@@ -0,0 +1,12 @@
+export function base64urlToBuffer(base64url) {
+  const base64 = base64url.replace(/-/g, "+").replace(/_/g, "/")
+  const padding = "=".repeat((4 - base64.length % 4) % 4)
+  const binary = atob(base64 + padding)
+  return Uint8Array.from(binary, c => c.charCodeAt(0)).buffer
+}
+
+export function bufferToBase64url(buffer) {
+  const bytes = new Uint8Array(buffer)
+  const binary = String.fromCharCode(...bytes)
+  return btoa(binary).replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/, "")
+}