[Feat]: Lock the codeserver interface after inactive timeperiod or manual logoff

## What is your suggestion?
We have a sign-out button that deletes the cookie file to clear the credentials. Can we have a logout menu that will lock the code server and asks for the password? The transparency of the lock screen can be made editable by user.
## Why do you want this feature?
There are some scenarios where i am not able to lock the system which will go to sleep as per my system settings. i need the laptop to be open + web browser active, but the code server can lock itself and show a password window after a specified window inactive period or when manually logged out.
This will definitely protect my code from anonymous modification and works well on shared computers or logins.
This happens when i train machine learning models on the server. it is mandatory that i should keep my system and browser open for several hours. But it becomes my prime responsibility that i should block someone from the same room views/access/modify/copy/download the code & results while running the builds.
signing out will stop me from viewing or reconnecting to the same instance to vire progress or results - especially in jupyter notebooks.
## Are there any workarounds to get this functionality today?
it must be developed based on the inactivity period or logoff menu
## Are you interested in submitting a PR for this?
No.

The payload names no files, tests, or entry points. Start by mapping the existing sign-out flow and checking whether inactivity handling already exists. Done would mean a lock screen can appear after configurable inactivity or manual logout, preserves the running session, and supports editable transparency.

Written by the indexing model from the issue text.