Feature Request: Add 'Rename User' option to Users & Groups settings

[sporteka2]

Problem

There is no way to rename a user account from the Cinnamon GUI. Renaming a user currently requires complex CLI steps that are error-prone.

Suggested Solution

Add a "Rename..." button in Users and Groups (cinnamon-settings-users) that:

1. Validates the new username
2. Runs usermod/groupmod behind the scenes
3. Offers to rename and migrate the home directory
4. Checks for and warns about running processes, open sessions, Docker, etc.
5. Updates /etc/subuid, /etc/subgid, crontabs, systemd user units automatically

Why it matters

• Switching display managers can break username-based session detection
• Users want to simplify their username
• Current CLI approach (usermod + groupmod + manual fixes) is fragile and intimidating for non-technical users