Lock the pref in modules/libpref/init/all.js so end users cannot toggle it from about:config or user.js, and add it to the Preferences enterprise-policy allowlist so admins can still drive it from policies.json. The pref stays declared as a StaticPref in StaticPrefList.yaml; this patch only adds the locking and policy plumbing. Differential Revision: https://phabricator.services.mozilla.com/D297713