Proxy /demo/ and /health through nginx to backend

nginx only proxied /api/ — so /demo/status returned the SPA HTML,
causing useDemoMode() to always resolve to false and all demo guards
to silently fail. Add /demo/ and /health proxy blocks.

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

frontend/nginx.conf

@@ -17,6 +17,20 @@ server {
         proxy_send_timeout 120s;
     }
 
+    # Proxy non-versioned backend endpoints (health, demo status)
+    location /demo/ {
+        proxy_pass http://backend:8000;
+        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;
+    }
+
+    location /health {
+        proxy_pass http://backend:8000;
+        proxy_set_header Host $host;
+    }
+
     # All other routes → index.html (React SPA)
     location / {
         try_files $uri $uri/ /index.html;