style(lists): apply gradient PageHeader to client/interest/yacht/company/berth lists

Pulls the polished gradient hero strip into the five primary list
surfaces. PR10b-e (detail polish, dashboard/admin polish, email +
notifications polish, mobile responsive sweep) deferred to a follow-up
release per spec risk register since visual baseline regen needs hands-
on iteration.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/companies/company-list.tsx

@@ -73,6 +73,7 @@ export function CompanyList() {
       <PageHeader
         title="Companies"
         description="Manage company records"
+        variant="gradient"
         actions={
           <PermissionGate resource="companies" action="create">
             <Button size="sm" onClick={() => setCreateOpen(true)}>