feat: self-service resume of an existing registration after refresh (S-26, closes #111) (#119)

## What & why

After submitting, the self-service portal held the registration only in in-memory signals, so a **page refresh stranded an in-flight registration** — the reference and its "Documenten aanleveren" / "Trek aanvraag in" actions were lost, with no way back (the reference wasn't in the URL and there was no read endpoint). This is the gap a citizen hit in testing.

Now the portal **resumes on load**:
- **Domain:** `IRegistrationStore.FindOpenByBsnAsync` (the citizen's non-terminal INGEDIEND/IN_BEHANDELING registration) + `GET /registrations/current?bsn=`.
- **BFF:** owner-scoped `GET /self-service/registrations` (bsn from the DigiD token) → the current registration, or **204** when none. Regenerated `services/bff/openapi.json`.
- **Frontend:** `registration-page` calls it on init and restores the submitted view (reference + actions); 204 shows the submit form as before. api-client regenerated (orval).

Closes #111

## Definition of Done

- [x] Linked issue (#111).
- [x] TDD — store `FindOpenByBsnAsync` tests, BFF endpoint tests, an Angular component test (resume-on-load), a Playwright e2e (submit → reload → restored).
- [x] Conventional Commits referencing #111.
- [ ] CI green — validated locally (below); runner CI running.
- [x] `docker compose up` reaches green health — fresh stack + full e2e (3 specs) green.
- [x] Docs — `docs/synthetic-data.md` (new e2e users).
- [ ] ADR — N/A (follows existing BFF/domain patterns; no boundary change).
- [ ] Demo note — the flow is unchanged for the demo; no new demo-script section (happy to add one if wanted).

## Verified locally

- Unit: Big 141 (+7 store tests), Bff 36 (+3 endpoint tests), all suites green.
- Frontend: 12 self-service component tests (incl. resume-on-load); lint + build green.
- **e2e (fresh CI stack): all 3 specs pass** — `registration`, `resume`, `withdrawal` (29.5s, single worker).
- Mutation: domain **91.04%**, bff **100%** (break 90%). `make lint` clean.

## Notes for reviewers

- **Shared-stack isolation:** resume-on-load restores any open registration for the logged-in bsn, so the self-service e2e specs can no longer share `jan-burger` (the verify-* API checks submit as `jan-burger`/`123456782` before the e2e). Each spec now has its own DigiD citizen (`emma`/`sanne`/`lars`-burger); `jan-burger` stays the documented citizen for the verify checks. This is the fix for the two intermittent e2e failures seen during development.
- **Scope:** resumes the current **in-flight** registration only (terminal ones aren't resumed), per the issue's out-of-scope note.

Reviewed-on: #119

apps/self-service/src/app/registration/registration-page.ts

@@ -1,5 +1,5 @@
-import { Component, inject, signal } from '@angular/core';
-import { BffApiV1Service, type SubmitAccepted } from 'api-client';
+import { Component, inject, type OnInit, signal } from '@angular/core';
+import { BffApiV1Service, type CurrentRegistration, type SubmitAccepted } from 'api-client';
 import { AuthService } from 'auth';
 import { UtrechtComponentsModule } from 'ui';
 
@@ -8,13 +8,16 @@ import { UtrechtComponentsModule } from 'ui';
  * registration. The bsn comes from the DigiD token (not a form field), so this is a confirm-and-
  * submit flow that posts to the BFF and shows the returned reference (ADR-0010; S-08c). After
  * submitting they can withdraw it — "trek aanvraag in" — keyed by that reference (S-11c).
+ *
+ * On load it asks the BFF for the caller's current open registration and restores the submitted view
+ * if there is one, so a page refresh no longer strands an in-flight registration (S-26).
  */
 @Component({
   selector: 'app-registration-page',
   imports: [UtrechtComponentsModule],
   templateUrl: './registration-page.html',
 })
-export class RegistrationPage {
+export class RegistrationPage implements OnInit {
   private readonly auth = inject(AuthService);
   private readonly bff = inject(BffApiV1Service);
 
@@ -31,6 +34,23 @@ export class RegistrationPage {
   protected readonly provideDocumentsFailed = signal(false);
   protected readonly selectedFile = signal<File | undefined>(undefined);
 
+  /** Resume an existing in-flight registration after a refresh (S-26): the BFF returns the caller's
+   * current open registration, or 204 (empty body) when there is none — in which case we show the
+   * submit form as before. Failures are non-fatal for the same reason. */
+  ngOnInit(): void {
+    this.bff.getSelfServiceRegistrations().subscribe({
+      next: (current: CurrentRegistration | void) => {
+        if (current && current.registrationId) {
+          this.reference.set(current.registrationId);
+          this.submitted.set(true);
+        }
+      },
+      error: () => {
+        // No resumable registration (or the lookup failed) — fall back to the submit form.
+      },
+    });
+  }
+
   submit(): void {
     this.submitting.set(true);
     this.failed.set(false);