WebKitGTK crashes at startup when its bubblewrap sandbox can't create an
unprivileged user namespace (bwrap: setting up uid map: Permission denied
-> Failed to fully launch dbus-proxy -> panic in webkit_web_view_load_uri).
This happens in containers/VMs and on Ubuntu 24.04+ where AppArmor
restricts unprivileged user namespaces. Detect that the kernel blocks
userns via procfs and set WEBKIT_DISABLE_SANDBOX_THIS_IS_DANGEROUS so the
UI stays usable; honor an explicit user override either way.
5bebecc427