feat: show identity match hint on registration success page

When a pending identity match is detected after volunteer registration,
the API now returns has_existing_account in the response. The success
page shows a login suggestion card so the volunteer can link their
registration to their existing account.

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

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

@@ -22,10 +22,15 @@ export function useRegistrationData(eventSlug: Ref<string>) {
   })
 }
 
+export interface VolunteerRegistrationResponse {
+  person: Record<string, unknown>
+  has_existing_account: boolean
+}
+
 export function useSubmitRegistration() {
   return useMutation({
     mutationFn: async ({ eventId, form }: { eventId: string; form: VolunteerRegistrationForm }) => {
-      const { data } = await apiClient.post<ApiResponse<Record<string, unknown>>>(
+      const { data } = await apiClient.post<ApiResponse<VolunteerRegistrationResponse>>(
         `/events/${eventId}/volunteer-register`,
         form,
       )