feat(portal): multi-step volunteer registration form with public event endpoint

- Add GET /api/v1/public/events/{slug}/registration-data endpoint for fetching
  event sections and time slots without auth
- Create 5-step registration form: personal info, details, motivation, section
  preferences, availability
- VeeValidate + Zod validation per step with Dutch error messages
- Auth-aware: pre-fills name/email for authenticated users
- Mobile responsive with custom chip-based step indicator
- Success page with contextual actions (dashboard vs login)
- Types, composable (TanStack Query), and Zod schemas

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/portal/src/composables/api/useVolunteerRegistration.ts

@@ -0,0 +1,36 @@
+import { useQuery, useMutation } from '@tanstack/vue-query'
+import type { Ref } from 'vue'
+import { apiClient } from '@/lib/axios'
+import type { EventRegistrationData, VolunteerRegistrationForm } from '@/types/registration'
+
+interface ApiResponse<T> {
+  data: T
+}
+
+export function useRegistrationData(eventSlug: Ref<string>) {
+  return useQuery({
+    queryKey: ['registration-data', eventSlug],
+    queryFn: async () => {
+      const { data } = await apiClient.get<ApiResponse<EventRegistrationData>>(
+        `/public/events/${eventSlug.value}/registration-data`,
+      )
+
+      return data.data
+    },
+    enabled: () => !!eventSlug.value,
+    retry: false,
+  })
+}
+
+export function useSubmitRegistration() {
+  return useMutation({
+    mutationFn: async ({ eventId, form }: { eventId: string; form: VolunteerRegistrationForm }) => {
+      const { data } = await apiClient.post<ApiResponse<Record<string, unknown>>>(
+        `/events/${eventId}/volunteer-register`,
+        form,
+      )
+
+      return data.data
+    },
+  })
+}