Biometric authentication was failing in Flatpak with the error "Unix process
subject does not have uid set". This occurred because polkit could not validate
the sandboxed PID against the host PID namespace.
Use polkit's system-bus-name subject type instead of unix-process. This allows
polkit to query D-Bus for the connection owner's host PID and credentials,
bypassing the PID namespace issue. Includes fallback to unix-process for edge
cases where D-Bus unique name is unavailable.
2e284c5e5a