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/render/merge.py

@@ -69,7 +69,7 @@ def parse_sidecar(path: Path) -> tuple[dict, str]:
     return {}, text.strip()
 
 
-def apply_sidecar(detail: dict, fm: dict, body: str) -> dict:
+def apply_sidecar(detail: dict, fm: dict, body: str, *, download_disabled_default: bool = False) -> dict:
     """Apply sidecar overrides to a detail JSON dict. Returns a modified copy."""
     from bincio.extract.writer import _infer_indoor_title
     d = dict(detail)
@@ -97,8 +97,12 @@ def apply_sidecar(detail: dict, fm: dict, body: str) -> dict:
         d["privacy"] = "unlisted" if fm["private"] else detail.get("privacy", "public")
     if "hide_stats" in fm:
         d["custom"]["hide_stats"] = [str(s) for s in (fm["hide_stats"] or [])]
-    if "download_disabled" in fm:
+    dd = fm.get("download_disabled")  # True, False, or None (absent)
+    if dd is True:
         d["download_disabled"] = True
+    elif dd is None and download_disabled_default:
+        d["download_disabled"] = True
+    # dd is False → explicit per-activity opt-in, leave unset
 
     return d
 
@@ -239,6 +243,13 @@ def _merge_all_locked(data_dir: Path) -> int:
     merged_dir = data_dir / "_merged"
     merged_acts = merged_dir / "activities"
 
+    _settings_path = data_dir / "_user_settings.json"
+    try:
+        _user_settings = json.loads(_settings_path.read_text(encoding="utf-8")) if _settings_path.exists() else {}
+    except (OSError, json.JSONDecodeError):
+        _user_settings = {}
+    _dl_default: bool = bool(_user_settings.get("download_disabled_default", False))
+
     # Collect sidecars upfront
     sidecars: dict[str, tuple[dict, str]] = {}
     if edits_dir.exists():
@@ -287,9 +298,9 @@ def _merge_all_locked(data_dir: Path) -> int:
                 detail = json.loads(src.read_text(encoding="utf-8"))
                 if activity_id in sidecars:
                     fm, body = sidecars[activity_id]
-                    detail = apply_sidecar(detail, fm, body)
+                    detail = apply_sidecar(detail, fm, body, download_disabled_default=_dl_default)
                 else:
-                    detail = apply_sidecar(detail, {}, "")
+                    detail = apply_sidecar(detail, {}, "", download_disabled_default=_dl_default)
                 if activity_id in image_lists:
                     detail["custom"] = dict(detail.get("custom") or {})
                     detail["custom"]["images"] = image_lists[activity_id]