feat(website): localize whitelabel partner cards (da/en)

Partner demo cards in section 06 were hardcoded Danish strings, so they
stayed Danish in EN mode. Move name + subtitle into COPY.whitelabel.partners
for both languages and render them via a mapped computed; per-card accent and
the placeholder style remain presentational config in the component.

Also harden PartnerCard's avatar-initial against an empty name to satisfy
noUncheckedIndexedAccess.

apps/website/components/landing/PartnerCard.vue

@@ -12,7 +12,7 @@ const props = withDefaults(defineProps<{
   placeholder?: boolean
 }>(), { placeholder: false })
 
-const initial = computed(() => props.name[0].toUpperCase())
+const initial = computed(() => (props.name[0] ?? '').toUpperCase())
 </script>
 
 <template>