feat: add create organisation button and dialog on platform page

Add "Nieuwe organisatie" button to the platform organisations list page.
Dialog with name field (auto-generates slug) and slug field. Uses the
existing POST /organisations endpoint. On success, navigates to the
new organisation's detail page.

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

apps/app/src/composables/api/useAdmin.ts

@@ -7,6 +7,7 @@ import type {
   AdminOrganisationDetail,
   AdminOrganisationMember,
   AdminUser,
+  CreateOrganisationPayload,
   ImpersonationResponse,
   InviteMemberPayload,
   PlatformStats,
@@ -77,6 +78,23 @@ export function useUpdateAdminOrganisation() {
   })
 }
 
+export function useCreateOrganisation() {
+  const queryClient = useQueryClient()
+
+  return useMutation({
+    mutationFn: async (payload: CreateOrganisationPayload) => {
+      const { data } = await apiClient.post<ApiResponse<AdminOrganisation>>(
+        '/organisations',
+        payload,
+      )
+      return data.data
+    },
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: ['admin', 'organisations'] })
+    },
+  })
+}
+
 export function useDeleteAdminOrganisation() {
   const queryClient = useQueryClient()