When running as a systemd service, the binary is often in /usr/local/bin
but WorkingDirectory points to a data directory like /var/lib/vikunja.
Previously, rootpath defaulted to the binary's directory, causing
Vikunja to attempt creating files/db in /usr/local/bin.
Now os.Getwd() is preferred, which respects systemd's WorkingDirectory
and is the intuitive default for all deployment scenarios.