fix(vscode): move extensions under profiles.default

programs.vscode.extensions was renamed to
programs.vscode.profiles.default.extensions in home-manager. The bare
option still worked via the compat shim but emitted a deprecation
warning on every eval (21× — once per home evaluation in flake check)
and will break when the alias is dropped. userSettings already lived
under profiles.default; extensions now joins it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

features/apps/vscode.nix

@@ -108,7 +108,8 @@ in
     programs.vscode = {
       enable = lib.mkDefault true;
       package = lib.mkDefault pkgs.vscode;
-      profiles.default.userSettings = lib.mkDefault {
+      profiles.default = {
+        userSettings = lib.mkDefault {
           "update.mode" = "none";
           "workbench.colorTheme" = themeConfig.name;
           "window.titleBarStyle" = "custom";
@@ -121,4 +122,5 @@ in
         );
       };
     };
+  };
 }