feat: add WebAuthn Experience API functions to sign-in UI

Adds startWebAuthnAuth and verifyWebAuthnAuth functions that call
the Logto Experience API WebAuthn endpoints for passkey MFA verification.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/sign-in/src/experience-api.ts

@@ -273,3 +273,28 @@ export async function submitMfa(verificationId: string): Promise<string> {
   await identifyUser(verificationId);
   return submitInteraction();
 }
+
+// --- WebAuthn MFA Verification ---
+
+export async function startWebAuthnAuth(): Promise<Record<string, unknown>> {
+  const res = await request('POST', '/verification/web-authn/authentication');
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({}));
+    throw new Error(err.message || `Failed to start passkey authentication (${res.status})`);
+  }
+  const data = await res.json();
+  return data;
+}
+
+export async function verifyWebAuthnAuth(payload: Record<string, unknown>): Promise<string> {
+  const res = await request('POST', '/verification/web-authn/authentication/verify', payload);
+  if (!res.ok) {
+    const err = await res.json().catch(() => ({}));
+    if (res.status === 422) {
+      throw new Error('Passkey verification failed. Please try again.');
+    }
+    throw new Error(err.message || `Passkey verification failed (${res.status})`);
+  }
+  const data = await res.json();
+  return data.verificationId;
+}