site/admin-ui: relative asset base for single-origin path prefix

Use base './' so built index.html references ./assets/... instead of
/assets/..., letting the SPAs work under the Easy Launcher's
single-origin path prefix (https://<apex>/__dn/admin/) as well as the
subdomain layout. Root-absolute paths would skip the prefix and 404.

DorkNet.Server/site/vite.config.ts

@@ -8,7 +8,10 @@ import path from 'node:path';
 // any extra publish wiring.
 export default defineConfig({
   plugins: [react()],
-  base: '/',
+  // Relative-base — see admin-ui/vite.config.ts. Same single-origin
+  // path-prefix scenario applies if the site ever gets opened through
+  // `https://<apex>/__dn/www/`.
+  base: './',
   resolve: {
     alias: { '@': path.resolve(__dirname, 'src') },
   },