fix(cibg): wizard fields render as grey fieldset groups

Wizard steps rendered bare .form-group divs, so CIBG's
".form-horizontal fieldset { background:#f1f5f9; margin-bottom:1.25em }"
never matched and inputs showed on white instead of the grey CIBG surface.

Wrap each logical field group per step in a <fieldset> (intake, herregistratie
and registratie wizards); CIBG then gives every group its grey surface with a
1.25em gap between groups. The shell stays group-agnostic (no outer fieldset,
which would hide the white gaps). address-fields already used a <fieldset>.

Adds intake-wizard.component.spec.ts asserting the buitenland step renders its
groups as separate fieldsets (guards against the wrapping being dropped again).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/shared/layout/wizard-shell/wizard-shell.component.ts

@@ -90,6 +90,10 @@ export type WizardStatus = 'editing' | 'submitting' | 'submitted' | 'failed';
               <span class="meta" i18n="@@form.verplichteVelden">* verplichte velden</span>
             </div>
           </div>
+          <!-- Wizard pages wrap their field groups in <fieldset>s; CIBG's
+               ".form-horizontal fieldset" gives each a grey #f1f5f9 surface with a 1.25em
+               gap. The shell stays group-agnostic and does NOT add its own fieldset (an
+               outer grey fieldset would hide the white gaps between the page groups). -->
           <ng-content />
           <hr />
           <div class="d-flex flex-column flex-sm-row-reverse">