Add Better Auth admin authentication

lib/auth-server.ts

@@ -0,0 +1,14 @@
+import { convexBetterAuthNextJs } from "@convex-dev/better-auth/nextjs";
+
+export const {
+  handler,
+  preloadAuthQuery,
+  isAuthenticated,
+  getToken,
+  fetchAuthQuery,
+  fetchAuthMutation,
+  fetchAuthAction,
+} = convexBetterAuthNextJs({
+  convexUrl: process.env.NEXT_PUBLIC_CONVEX_URL!,
+  convexSiteUrl: process.env.NEXT_PUBLIC_CONVEX_SITE_URL!,
+});