Migrate the viewer build off the deprecated browser builder #210

Symptom

npm run build warns that the Webpack browser builder is deprecated.

The origin/dev build target in angular.json is @angular-builders/custom-webpack:browser (@angular-builders/custom-webpack 22.0.1). That builder calls executeBrowserBuilder from @angular-devkit/build-angular. The browser builder logs:

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

@angular-builders/custom-webpack 22.0.1 exposes browser, dev-server, server, karma, and extract-i18n. It has no application builder.

Why this is not on the LTR pointer PR

Left out of sitmun/sitmun-application-stack#97 on purpose. That PR is the AG Grid, language, and cartography follow-up (admin 8bcd38f4, backend 374e1bbe, stack c645b873). This is a separate build migration.

Acceptance

• The production build no longer goes through @angular-devkit/build-angular:browser, and npm run build does not print that deprecation.
• webpack.config.js behavior still holds: the Cesium @spz-loader/core alias, Babel excludes for jszip and util, dev-server HMR off, the /catastro-inspire proxy, the process fallback, and the api-sitna copy with SITNA_BASE_URL.
• Because custom-webpack 22 has no application builder, those customizations need an @angular/build:application equivalent. Renaming the current target is not enough.