feat: add passkey-first login and management flow

src/utils/passkey.ts

@@ -0,0 +1,30 @@
+export function bytesToBase64Url(bytes: Uint8Array): string {
+  let binary = '';
+  for (const b of bytes) binary += String.fromCharCode(b);
+  return btoa(binary).replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/g, '');
+}
+
+export function base64UrlToBytes(input: string): Uint8Array {
+  const normalized = String(input || '').replace(/-/g, '+').replace(/_/g, '/');
+  const padded = normalized + '='.repeat((4 - (normalized.length % 4 || 4)) % 4);
+  const binary = atob(padded);
+  const out = new Uint8Array(binary.length);
+  for (let i = 0; i < binary.length; i += 1) out[i] = binary.charCodeAt(i);
+  return out;
+}
+
+export function randomChallenge(size: number = 32): string {
+  return bytesToBase64Url(crypto.getRandomValues(new Uint8Array(size)));
+}
+
+export function parseClientDataJSON(base64Url: string): { type?: string; challenge?: string; origin?: string } | null {
+  try {
+    const raw = base64UrlToBytes(base64Url);
+    const text = new TextDecoder().decode(raw);
+    const parsed = JSON.parse(text) as { type?: string; challenge?: string; origin?: string };
+    if (!parsed || typeof parsed !== 'object') return null;
+    return parsed;
+  } catch {
+    return null;
+  }
+}