feat(portal-self-service): implement the DigiD registration submit page (refs #67)

RegistrationPage shows the signed-in BSN and submits to the BFF via the generated
api-client, confirming with the returned reference; built from NL Design System
(Utrecht) components. Wire the guarded route + app providers (DigiD OIDC + token
interceptor + HttpClient), the NL DS theme, and lang=nl. Component tests
(Testing Library) + axe (WCAG 2.1 AA) pass; a guard test covers libs/auth. Replace
the demo eslint depConstraints (scope:shop/shared) with a permissive default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/self-service/project.json

@@ -27,8 +27,8 @@
           "budgets": [
             {
               "type": "initial",
-              "maximumWarning": "500kb",
-              "maximumError": "1mb"
+              "maximumWarning": "1mb",
+              "maximumError": "2mb"
             },
             {
               "type": "anyComponentStyle",