docs(portal-self-service): record NL DS + DigiD decisions and demo note (refs #67)

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

docs/frontend-decisions.md

@@ -48,3 +48,27 @@ with the submit form (S-08c, #67); any deviation from NL DS will be recorded her
 - The BFF endpoints carry no `operationId`, so orval synthesises method names
   (`postSelfServiceRegistrations`, `getOpenbaarRegister`); adding explicit operation ids to the BFF
   is a possible later polish.
+
+---
+
+## Self-service form: NL DS, DigiD auth, testing (S-08c, #67)
+
+- **NL Design System via `@utrecht/component-library-angular`** (`libs/ui`) + `@utrecht/design-tokens`
+  (imported once in `apps/self-service/src/styles.css`). Utrecht is NL DS's reference Angular
+  implementation. Its v3 components are **NgModule-based, not standalone**, so `libs/ui` re-exports
+  `UtrechtComponentsModule` (and the component classes, so the AOT compiler resolves the template
+  directives through the barrel); standalone components consume it via `imports: [UtrechtComponentsModule]`.
+  §10's "no NgModules in new code" governs *our* code — consuming a third-party module is fine.
+- **DigiD login via `angular-auth-oidc-client`** (`libs/auth`): auth-code + PKCE against the Keycloak
+  `digid` realm (public client `big-portal`). A small **`AuthService` abstraction** (bsn /
+  isAuthenticated / login) wraps the library so components and the `authenticatedGuard` depend on a
+  mockable surface; a **token `HttpInterceptor`** attaches the bearer to BFF calls (secure route).
+  The OIDC `authority`/`secureApiOrigin` are dev defaults in `app.config.ts`; the compose-served app
+  overrides them (S-08d), and the browser-vs-container issuer alignment is handled there (ADR-0010).
+- **Testing:** component tests use **`@testing-library/angular`** (§10) with `AuthService` and the
+  api-client mocked; the **axe** (`vitest-axe`) check runs scoped to WCAG 2.1 AA tags
+  (`wcag2a/2aa/21a/21aa`) with the document `lang` set, asserting zero violations on the submit page.
+  The real DigiD browser round-trip is exercised in S-08d (Playwright).
+- **Module boundaries:** replaced the demo eslint `depConstraints` (`scope:shop`/`scope:shared`, left
+  over from the Nx angular template) with a permissive `*` default; scope/type tags can be
+  introduced when the portal set grows.