New pref download_disabled_default (stored in user_prefs + mirrored to
_user_settings.json for the render pipeline). When true, apply_sidecar
marks all activities as download_disabled unless the sidecar explicitly
sets download_disabled: false (per-activity opt-in from the edit drawer).
Settings page gets an "Activity defaults" card with the toggle.
Davide Scaini
de602ff5d9