fix: minor system improvements

[jaeheonji]

This PR is a suggestion from the discord channel.

Summary

• /tmp/zellij-1000/zellij-log/zellij.log this file seems to have 700 permission on linux systems. I don't understand why the executable permission is required here. It can just have 600.
• I think that the No active zellij sessions found. message on running zellij attach command should go to stderr instead of stdout. Same with the zellij list-sessions command. It makes scripting easier e.g. zellij list-sessions | grep -qF "$session_name" . Ideally, there should be no output to stdout when no session is running.