feat(ux): shared PageTitle/SectionTitle primitive, adopt app-wide

- new components/ui/page-title.tsx (PageHeader/PageTitle/SectionTitle)
- every page heading routed through PageTitle (one source of truth
  for the page-title style; identical render)
- members restricted-directory screen now uses the standard
  PageHeader bar instead of an off-style mono heading

app/(app)/members/members-client.tsx

@@ -6,6 +6,7 @@ import { addMember, updateMember, approveMember, removeMember, assignAreaLead }
 import { toast } from 'sonner'
 import { useRouter } from 'next/navigation'
 import type { Tables } from '@/types/database'
+import { PageTitle } from '@/components/ui/page-title'
 
 type Member = Tables<'space_members'>
 type AreaLeadRole = { id: string; area_name: string; lead_id: string | null }
@@ -194,7 +195,7 @@ export function MembersClient({ members: initialMembers, currentRole, areaLeadRo
     <div className="min-h-screen bg-background">
       <div className="bg-sidebar px-4 md:px-6 py-3 flex items-center justify-between">
         <div className="flex items-center gap-3">
-          <h1 className="text-white font-sans text-lg font-semibold">Members</h1>
+          <PageTitle>Members</PageTitle>
           <span className="font-mono text-xs text-white/50">{total} total</span>
         </div>
         {isAdmin(currentRole) && (