Fix: Sanitize username to prevent single-instance detection failure

[sshekhar563]

Problem:

The single-instance detection feature fails on systems where the environment variable used for the username (USER or USERNAME) contains characters that are invalid in filenames (e.g., /, ).

This prevents the .lock file from being created, which causes the application to allow multiple instances to run simultaneously, even when the option is disabled.

Solution:

This PR resolves the issue by sanitizing the username string before it is used to construct the lock file identifier. It uses a regular expression to find and replace all invalid filename characters with an underscore (_).

This approach is more robust as it protects against invalid characters from any source, ensuring the lock file can always be created.