Cockpit: cockpit: local privilege escalation via insecure symlink handling in public key addition helper

A flaw was found in Cockpit. A local authenticated user with control over a target account's home directory can exploit a privileged symlink dereference vulnerability in the `ssh-add-public-key.sh` helper. When this helper runs with elevated privileges, it can be tricked into following attacker-controlled symlinks within the `.ssh` or `authorized_keys` paths. This allows the attacker to arbitrarily change file ownership and modes, potentially leading to local privilege escalation and impacting the integrity, confidentiality, and availability of the system.