fix: prevent stale chunk errors after deployments

- Set Cache-Control no-cache on index.html in nginx so browsers always
  fetch fresh chunk references after a deploy
- Auto-reload on chunk load failures (stale deploy detection) with
  loop prevention via sessionStorage
- Show friendly "App Updated" message if auto-reload doesn't resolve it

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

frontend/nginx.conf

@@ -8,12 +8,19 @@ server {
     gzip on;
     gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
 
+    # index.html — never cache (so deploys serve new chunk references)
+    location = /index.html {
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+        add_header Pragma "no-cache";
+        add_header Expires "0";
+    }
+
     # Handle SPA routing - serve index.html for all routes
     location / {
         try_files $uri $uri/ /index.html;
     }
 
-    # Cache static assets
+    # Cache hashed static assets (immutable — filenames change on rebuild)
     location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg|woff|woff2)$ {
         expires 1y;
         add_header Cache-Control "public, immutable";