Fix members routing - restructure as directory for detail page to work

Move members.tsx to members/index.tsx so $memberId.tsx resolves as a
sibling route. Fixes member detail page not loading when navigating
from members list.

packages/admin/src/routes/_authenticated/members/index.tsx

@@ -15,7 +15,7 @@ import {
 } from '@/components/ui/dropdown-menu'
 import { Search, Plus, MoreVertical, Pencil, Users } from 'lucide-react'
 
-export const Route = createFileRoute('/_authenticated/members')({
+export const Route = createFileRoute('/_authenticated/members/')({
   validateSearch: (search: Record<string, unknown>) => ({
     page: Number(search.page) || 1,
     limit: Number(search.limit) || 25,