feat(portal): sentence-case button labels on public register page

Vuexy's Vuetify preset capitalizes VBtn labels, so "Sla op als concept"
rendered as "Sla Op Als Concept". Dutch convention on this page is
sentence-case. Adds a scoped utility class applied to the four VBtns on
the public register page (Vorige / Sla op als concept / Volgende /
Verstuur) rather than touching the global Vuetify defaults.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/portal/src/pages/register/[public_token].vue

@@ -442,6 +442,7 @@ function formatReviewValue(field: PublicFormField): string {
               v-if="currentStep > 0"
               variant="outlined"
               color="secondary"
+              class="form-btn-sentence-case"
               @click="prevStep"
             >
               <VIcon
@@ -457,6 +458,7 @@ function formatReviewValue(field: PublicFormField): string {
                 v-if="activeStep && activeStep.kind !== 'review'"
                 variant="text"
                 color="primary"
+                class="form-btn-sentence-case"
                 :loading="draft.isSaving.value"
                 :disabled="!draft.submission.value"
                 @click="onSaveDraft"
@@ -467,6 +469,7 @@ function formatReviewValue(field: PublicFormField): string {
               <VBtn
                 v-if="currentStep < steps.length - 1"
                 color="primary"
+                class="form-btn-sentence-case"
                 :disabled="!isActiveStepValid"
                 @click="nextStep"
               >
@@ -480,6 +483,7 @@ function formatReviewValue(field: PublicFormField): string {
               <VBtn
                 v-else
                 color="success"
+                class="form-btn-sentence-case"
                 :loading="draft.isSubmitting.value"
                 @click="onSubmit"
               >
@@ -532,4 +536,10 @@ function formatReviewValue(field: PublicFormField): string {
 .public-form-page {
   min-block-size: 100dvh;
 }
+
+/* Portal theme capitalizes VBtn labels by default; this page uses
+   sentence-case Dutch labels ("Sla op als concept", not title-case). */
+.form-btn-sentence-case {
+  text-transform: none !important;
+}
 </style>