fix(portal-shell): make 'nx test' run once by default, add watch configuration

The Angular 21 unit-test builder (@angular/build:unit-test) defaults
to watch mode. Without an explicit option, 'pnpm nx test portal-shell'
hangs on 'Waiting for task' indefinitely - unsuitable for CI and
surprising for ad-hoc invocations.

Pin watch=false as the default in the target options. Add a 'watch'
configuration so developers who want continuous test running can opt
in with 'pnpm nx test portal-shell --configuration=watch'. portal-bff
uses Jest which defaults to no-watch and needs no change.

apps/portal-shell/project.json

@@ -64,7 +64,14 @@
     },
     "test": {
       "executor": "@angular/build:unit-test",
-      "options": {}
+      "options": {
+        "watch": false
+      },
+      "configurations": {
+        "watch": {
+          "watch": true
+        }
+      }
     },
     "serve-static": {
       "continuous": true,