🔒 Fix Command Execution with relative useradd path

[Cylae]

🎯 What:
Replaced Command::new("useradd") with absolute path /usr/sbin/useradd in server_manager/src/core/system.rs and added a unit test module validating username input sanitization.

⚠️ Risk:
Using relative executable names in Command::new relies on $PATH resolution. If an attacker gains control over $PATH or writes to a directory preceding /usr/sbin in $PATH, they could hijack execution when creating system users.

🛡️ Solution:
Updating Command::new("useradd") to /usr/sbin/useradd guarantees execution of the trusted system binary regardless of environment $PATH overrides.

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PR created automatically by Jules for task 13098466155646312693 started by @Cylae

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