Settings: per-user default for download_disabled

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.

bincio/edit/ops.py

@@ -50,8 +50,12 @@ def apply_sidecar_edit(activity_id: str, payload: dict[str, Any], data_dir: Path
     hide = [s for s in (payload.get("hide_stats") or []) if s in STAT_PANELS]
     if hide:
         lines.append(f"hide_stats: [{', '.join(hide)}]")
-    if payload.get("download_disabled"):
+    dd = payload.get("download_disabled")
+    if dd is True:
         lines.append("download_disabled: true")
+    elif dd is False:
+        # Explicit false: allows per-activity opt-in against a user-level default
+        lines.append("download_disabled: false")
 
     description = (payload.get("description") or "").strip()