chore: add mysql demo

demo/nextjs/lib/auth.ts

@@ -15,6 +15,7 @@ import { reactResetPasswordEmail } from "./email/rest-password";
 import { resend } from "./email/resend";
 import { MysqlDialect } from "kysely";
 import { createPool } from "mysql2/promise";
+import { nextCookies } from "better-auth/next-js";
 
 const from = process.env.BETTER_AUTH_EMAIL || "delivered@resend.dev";
 const to = process.env.TEST_EMAIL || "";
@@ -25,13 +26,15 @@ const libsql = new LibsqlDialect({
 });
 
 const mysql = new MysqlDialect(
-	createPool("mysql://user:password@localhost:3306/better_auth"),
+	createPool(process.env.MYSQL_DATABASE_URL || ""),
 );
 
+const dialect = process.env.USE_MYSQL ? mysql : libsql;
+
 export const auth = betterAuth({
 	appName: "Better Auth Demo",
 	database: {
-		dialect: mysql,
+		dialect,
 		type: "mysql",
 	},
 	session: {
@@ -141,5 +144,6 @@ export const auth = betterAuth({
 		multiSession(),
 		oneTap(),
 		oAuthProxy(),
+		nextCookies(),
 	],
 });