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/package.json

@@ -27,6 +27,7 @@
     "@tiptap/pm": "^2.27.1",
     "@tiptap/starter-kit": "^2.27.1",
     "@tiptap/vue-3": "^2.27.1",
+    "@vee-validate/zod": "^4.15.1",
     "@vueuse/core": "10.11.1",
     "@vueuse/math": "10.11.1",
     "apexcharts": "3.54.1",
@@ -45,6 +46,7 @@
     "swiper": "11.2.10",
     "ufo": "1.6.1",
     "unplugin-vue-define-options": "1.5.5",
+    "vee-validate": "^4.15.1",
     "vue": "3.5.22",
     "vue-chartjs": "5.3.2",
     "vue-flatpickr-component": "11.0.5",