🐛(frontend) return real 404s for missing hashed assets

Nginx was rewriting missing `/assets/*` files to `index.html`, so
requests for stale chunks got a 200 with the SPA shell served as a
JS module. That is what turned a plain 404 into the confusing:

  "TypeError: error loading dynamically imported module"

whenever a client loaded before a deployment requested an old chunk.

Stop rewriting missing `/assets/*` to `index.html`: those requests
now return a real 404 with `Cache-Control: no-store`. SPA route
fallback for non-asset paths is unchanged.

docker/dinum-frontend/nginx/default.conf

@@ -74,6 +74,13 @@ server {
   location ~* ^/assets/.*\.(css|js|json|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
       expires 30d;
       add_header Cache-Control "public, max-age=2592000";
+      try_files $uri =404;
+      error_page 404 = @asset_missing;
+  }
+
+  location @asset_missing {
+      add_header Cache-Control "no-store" always;
+      return 404;
   }
 
   # Serve static files

src/frontend/default.conf

@@ -14,6 +14,13 @@ server {
   location ~* ^/assets/.*\.(css|js|json|png|jpg|jpeg|gif|ico|svg|woff|woff2|ttf|eot)$ {
       expires 30d;
       add_header Cache-Control "public, max-age=2592000";
+      try_files $uri =404;
+      error_page 404 = @asset_missing;
+  }
+
+  location @asset_missing {
+      add_header Cache-Control "no-store" always;
+      return 404;
   }
 
   # Serve static files