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>

docker/nginx.conf

@@ -67,6 +67,28 @@ http {
         }
     }
 
+    # Marketing site — corrosionmgmt.com (bare domain)
+    server {
+        listen 80;
+        server_name corrosionmgmt.com;
+
+        # Early access signup API
+        location /api/early-access/ {
+            limit_req zone=api burst=10 nodelay;
+            proxy_pass http://api;
+            proxy_set_header Host $host;
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header X-Forwarded-Proto $scheme;
+        }
+
+        # SPA
+        location / {
+            root /usr/share/nginx/html;
+            try_files $uri $uri/ /index.html;
+        }
+    }
+
     # Wildcard server — *.corrosionmgmt.com (public server sites)
     server {
         listen 80;