feat: Domain-based routing — marketing site at bare domain, panel at subdomain

corrosionmgmt.com now serves LandingView as the default page with marketing
routes at root level. panel.corrosionmgmt.com continues serving the admin
panel unchanged. /site/* backward compat via redirects on marketing domain.

- nginx: Add bare domain server block (only proxies /api/early-access/)
- router: Detect hostname at module load, generate domain-specific routes
- MarketingLayout: Named routes for nav, external <a> tags for auth links
- LandingView: CTAs point to panel domain via VITE_PANEL_URL

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

.env.example

@@ -39,3 +39,6 @@ SMTP_FROM=noreply@corrosionmgmt.com
 # Server
 API_PORT=3000
 FRONTEND_URL=http://localhost:5174
+
+# Frontend (Vite — must be prefixed with VITE_)
+VITE_PANEL_URL=https://panel.corrosionmgmt.com