feat: implement credentials login

implements the credentials login functionality

src/validation.ts

@@ -51,7 +51,7 @@ export const registerSchema = zod
       !data.password.includes(data.username),
     {
       message:
-        'Password cannot contain your first name, last name, email, or username',
+        'Password cannot contain your first name, last name, email, or username itself',
       path: ['password'],
     },
   );