fix: disabling auth wipes all users' preferences on next pref save (#1764)

routes/prefs_routes.py

@@ -40,7 +40,18 @@ def _save_for_user(user: Optional[str], prefs: dict):
     """Save preferences for a specific user."""
     all_prefs = _load()
     if user is None:
-        # Auth disabled — save flat
+        # Auth disabled. If the store is already multi-user (e.g. auth was
+        # turned off on a deployment that previously ran multi-user), writing
+        # `prefs` flat would overwrite the whole `_users` map and destroy every
+        # other user's preferences. Instead write back into the same (first)
+        # slot _load_for_user(None) reads from, preserving the others.
+        if "_users" in all_prefs:
+            users = all_prefs["_users"]
+            first_key = next(iter(users), None)
+            if first_key is not None:
+                users[first_key] = prefs
+                _save(all_prefs)
+                return
         _save(prefs)
         return
     if "_users" not in all_prefs: