feat(cinnamon-settings-users): add user account rename support

[farhan11166]

Summary

Adds a Rename... action to Cinnamon Users and Groups.

The rename workflow:

• Validates the new username
• Checks for duplicate usernames
• Warns about running processes and open sessions
• Renames the login with usermod
• Migrates the home directory
• Renames a same-named private group
• Updates /etc/subuid and /etc/subgid
• Migrates the user crontab
• Refreshes the users and groups lists
• Displays errors when system commands fail

Testing

• Python syntax check passed
• GTK UI XML validation passed
• git diff --check passed
• Runtime testing on a real Cinnamon installation is pending(APolgies will try that in a weeks time my systems can take load of an addtional distro tried on vm but was lagging will do asap if possible anyone could check would be glad shall msotly work as i didnt meddle with other features jsut added an addtional one).

Known limitations

• Docker configuration is not automatically migrated
• Systemd user-unit configuration is not automatically rewritten
• Arbitrary references to the old username in configuration files are not changed
• Full runtime testing is still required with a disposable user account

This resolves majority of #13866 except the limitations mentioned above which could be implemented iteratively once this is merged