fix: ArrayBuffer type casts for WebAuthn ceremony in TS 5.x

Uint8Array.buffer returns ArrayBufferLike which is incompatible with
PublicKeyCredentialCreationOptions/RequestOptions in strict mode.

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

src/app/auth/signin/page.tsx

@@ -64,13 +64,13 @@ function SignInForm() {
       const challenge = Uint8Array.from(atob(options.challenge.replace(/-/g, "+").replace(/_/g, "/")), c => c.charCodeAt(0));
 
       const publicKeyOptions: PublicKeyCredentialRequestOptions = {
-        challenge,
+        challenge: challenge.buffer as ArrayBuffer,
         rpId: options.rpId,
         userVerification: options.userVerification as UserVerificationRequirement,
         timeout: options.timeout,
         allowCredentials: options.allowCredentials?.map((c: { type: string; id: string; transports?: string[] }) => ({
           type: c.type as PublicKeyCredentialType,
-          id: Uint8Array.from(atob(c.id.replace(/-/g, "+").replace(/_/g, "/")), ch => ch.charCodeAt(0)),
+          id: Uint8Array.from(atob(c.id.replace(/-/g, "+").replace(/_/g, "/")), ch => ch.charCodeAt(0)).buffer as ArrayBuffer,
           transports: c.transports as AuthenticatorTransport[],
         })),
       };

src/app/auth/signup/page.tsx

@@ -94,10 +94,10 @@ export default function SignUpPage() {
       }
 
       const publicKeyOptions: PublicKeyCredentialCreationOptions = {
-        challenge: fromBase64url(options.challenge),
+        challenge: fromBase64url(options.challenge).buffer as ArrayBuffer,
         rp: options.rp,
         user: {
-          id: fromBase64url(options.user.id),
+          id: fromBase64url(options.user.id).buffer as ArrayBuffer,
           name: options.user.name,
           displayName: options.user.displayName,
         },