fix: update document.title on every page via useEffect

All pages are 'use client' so metadata exports don't work. Each page now
sets document.title via useEffect — static pages with a fixed string,
dynamic pages keyed on data so the title reflects the loaded content.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/groups/page.tsx

@@ -1,5 +1,6 @@
 'use client'
 import { useQuery, gql } from '@/lib/graphql/hooks'
+import { useEffect } from 'react'
 import Link from 'next/link'
 import { TeamFlag } from '@/components/team-flag'
 
@@ -22,6 +23,8 @@ interface Standing {
 export default function GroupsPage() {
   const { data, loading } = useQuery(GROUPS_QUERY, { pollInterval: 60_000 })
 
+  useEffect(() => { document.title = 'Group Stage · World Cup' }, [])
+
   const standings: Standing[] = data?.groupStandings ?? []
   const byGroup = standings.reduce<Record<string, Standing[]>>((acc, s) => {
     acc[s.groupName] = [...(acc[s.groupName] ?? []), s]