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/serve/routers/me.py

@@ -222,6 +222,18 @@ async def me_set_prefs(
     # Coerce all values to strings; ignore unknown keys silently
     prefs = {str(k): str(v) for k, v in body.items()}
     set_user_prefs(deps._get_db(), user.handle, prefs)
+
+    # Mirror download_disabled_default to a file so the render pipeline can read it
+    if "download_disabled_default" in prefs:
+        user_dir = deps._get_data_dir() / user.handle
+        settings_path = user_dir / "_user_settings.json"
+        try:
+            current = json.loads(settings_path.read_text(encoding="utf-8")) if settings_path.exists() else {}
+        except (OSError, json.JSONDecodeError):
+            current = {}
+        current["download_disabled_default"] = prefs["download_disabled_default"] == "true"
+        settings_path.write_text(json.dumps(current, indent=2), encoding="utf-8")
+
     return JSONResponse({"ok": True})