vscodium: disable automatic Nix formatting

it always tried to set the themes by itself at runtime
and then complained due to readonly settings

home/home.nix

@@ -345,9 +345,16 @@ in
     userSettings = {
       "editor.fontLigatures" = true;
       "window.autoDetectColorScheme" = true;
+      "workbench.preferredDarkColorScheme" = "Default Dark Modern";
+      "workbench.preferredLightColorScheme" = "Default Light Modern";
       "[nix]" = {
         "editor.insertSpaces" = true;
         "editor.tabSize" = 2;
+        "editor.defaultFormatter" = "kamadorueda.alejandra";
+        # for now, disable automatic formatting to prevent disruption of existing code bases
+        "editor.formatOnPaste" = false;
+        "editor.formatOnSave" = false;
+        "editor.formatOnType" = false;
       };
       "editor.fontSize" = 13;
       "editor.fontWeight" = "normal";