fix(nginx): disable caching for index.html to prevent stale JS bundles

Without Cache-Control: no-cache on index.html, browsers serve old
cached HTML referencing outdated JS bundle hashes after deployment.

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

frontend/nginx.conf

@@ -14,8 +14,15 @@ server {
         proxy_set_header X-Forwarded-Proto $scheme;
     }
 
-    # SPA routing - all other routes serve index.html
+    # Cache static assets with content hash (JS, CSS) indefinitely
+    location ~* \.(js|css)$ {
+        try_files $uri =404;
+        add_header Cache-Control "public, max-age=31536000, immutable";
+    }
+
+    # SPA routing - index.html must never be cached
     location / {
         try_files $uri $uri/ /index.html;
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
     }
 }