feat(operator): create tenant from the operator UI

Wires the previously-dead 'New tenant' button on /tenants to a modal
that collects slug + name + plan + optional primary domain, POSTs to
the existing platform-api /tenants endpoint via a new operator proxy,
and navigates into the freshly-created tenant detail page. Slug
auto-derives from the name until the operator types in the slug field
themselves. Billing details and provisioning are still done from the
tenant detail page after creation — this modal is the minimum that
backend validators will accept.

apps/operator/pages/tenants/index.vue

@@ -7,6 +7,16 @@ const { data: tenants, refresh, pending } = await useFetch<Tenant[]>('/api/tenan
 
 const search = ref('')
 const statusFilter = ref<'all' | TenantStatus>('all')
+const createOpen = ref(false)
+
+async function onCreated(tenant: Tenant) {
+  createOpen.value = false
+  // Refresh the list so the new row appears immediately, then jump into the
+  // detail page — that's where the operator will configure domains, billing,
+  // and trigger provisioning.
+  await refresh()
+  await navigateTo(`/tenants/${tenant.slug}`)
+}
 
 const filtered = computed(() => {
   const q = search.value.trim().toLowerCase()
@@ -50,13 +60,15 @@ function navTo(t: Tenant) {
           <template #leading><UiIcon name="refresh" :size="13" /></template>
           Refresh
         </UiButton>
-        <UiButton variant="primary">
+        <UiButton variant="primary" @click="createOpen = true">
           <template #leading><UiIcon name="plus" :size="13" /></template>
           New tenant
         </UiButton>
       </template>
     </PageHeader>
 
+    <NewTenantModal :open="createOpen" @close="createOpen = false" @created="onCreated" />
+
     <div class="stage">
       <div class="filters">
         <div class="search">