refactor: extract toWizardErrors, adopted by all 3 wizards (RD-20)

Each wizard flattened its per-field error record into the shell's
WizardError[] summary with its own copy of the same loop. Extract one
pure helper, wizard-errors.ts, next to naarStapLabel. Add a spec that
covers a flat record, an empty record, skipped undefined/empty-string
values, the idPrefix, and a skipped nested object.

registratie-wizard.machine.ts changes Errors from an interface to a
type alias, because only a type alias gets an implicit index
signature and is assignable to the helper's Record<string, unknown>
parameter. The other two machines already declare their error maps as
type aliases, so this also makes the three consistent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/ssp/src/app/registratie/ui/registratie-wizard/registratie-wizard.component.ts

@@ -17,6 +17,7 @@ import {
   WizardPhase,
   naarStapLabel,
 } from '@shared/layout/wizard-shell/wizard-shell.component';
+import { toWizardErrors } from '@shared/layout/wizard-shell/wizard-errors';
 import { ASYNC } from '@shared/ui/async/async.component';
 import { AddressFieldsComponent } from '@registratie/ui/address-fields/address-fields.component';
 import { createStore } from '@shared/application/store';
@@ -468,14 +469,7 @@ export class RegistratieWizardComponent {
   /** Current step's errors (incl. per-question), flattened for the error summary. */
   protected errorList = computed<WizardError[]>(() => {
     const e = this.invullen()?.errors ?? {};
-    const out: WizardError[] = [];
-    for (const [k, v] of Object.entries(e)) {
-      if (k !== 'antwoorden' && typeof v === 'string' && v) out.push({ id: k, message: v });
-    }
-    for (const [qid, msg] of Object.entries(e.antwoorden ?? {})) {
-      if (msg) out.push({ id: 'vraag-' + qid, message: msg });
-    }
-    return out;
+    return [...toWizardErrors(e), ...toWizardErrors(e.antwoorden ?? {}, 'vraag-')];
   });
   protected adresSamenvatting = computed(() => {
     const d = this.draft();