Migrate the admin build off the deprecated browser builder #496

Symptom

angular.json on origin/dev and on 8bcd38f4 sets the build target to @angular-devkit/build-angular:browser. That builder warns:

The "@angular-devkit/build-angular:browser" builder is deprecated as part of Angular's Webpack support deprecation. Use "@angular/build:application" instead.

Admin does not use custom-webpack. The build target names the deprecated builder directly. The dev-server target is @angular-devkit/build-angular:dev-server.

Why this is not on the LTR pointer PR

Not part of sitmun/sitmun-application-stack#97. Admin 8bcd38f4 on that PR still uses the browser builder. The migration was left for a separate PR.

Acceptance

• The build target uses @angular/build:application, and the dev-server target uses the matching dev-server builder.
• npm run build does not warn about @angular-devkit/build-angular:browser.
• Current build options still apply: outputPath, assets, styles, and allowedCommonJsDependencies.