fix(public): always show real hero; drop nav from landing page

Bug 1 — Logged-out visitors saw the coming-soon Maintenance page while
admins saw the real hero. That difference is produced client-side by
MaintenanceGate (site_mode=maintenance && no user). Pull the `/` hero
route out from behind the gate so every visitor always lands on the real
Portal hero; the MaintenanceGate stays on all other public routes, so
content pages remain gated during maintenance and admins still preview
through it.

Bug 2 — The landing hero rendered the site nav because Portal used
PublicLayout with the default header=true. Pass header={false} so the
hero has no top nav (footer retained), using the layout's existing
escape hatch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0114TpmrNW4wNXsHq5CR72jQ

client/src/routes/public/Portal.jsx

@@ -40,7 +40,7 @@ export default function Portal() {
   const elements = [...layout.elements].sort((a, b) => (a.z || 0) - (b.z || 0))
 
   return (
-    <PublicLayout>
+    <PublicLayout header={false}>
       <main style={{ minHeight: '100vh', display: 'flex', flexDirection: 'column' }}>
         {PREVIEW && draft && (
           <div