Update Security and 2FA Images

Author: praveen-dp1412

docs/Security/Security.md

@@ -72,7 +72,7 @@ Once 2FA is activated, Doculan automatically generates **10 backup codes**.
 
 ## Code Expiration
 
-- Each 2FA code generated by the authenticator app **expires every 12 hours**.
+- Each 2FA code generated by the authenticator app becomes **invalid after logout** and must be regenerated for the **next login session**.
 - Once a code expires, open your authenticator app to obtain a **fresh 6-digit code**.
 - As an alternative, you can use one of your **backup codes** to access your account.