Move SSR server bundle to private directory

- Configure webpack to output server bundle to app/assets/builds
- Update React on Rails config to use new server bundle location
- Add app/assets/builds to .gitignore to keep server bundles private
- Server bundles are now isolated from public webpack output

This follows React on Rails 16 best practices for server-side rendering
by keeping server bundles in a private directory separate from client assets.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: justin808

.gitignore

@@ -31,6 +31,7 @@ vendor/bundle
 # Generated js bundles
 /public/packs
 /public/packs-test
+/app/assets/builds
 
 # Rubymine/IntelliJ
 .idea

config/initializers/react_on_rails.rb

@@ -12,7 +12,7 @@
   # If you are never using server rendering, you may set this to "".
   # If you are using the same file for client and server rendering, having this set probably does
   # not affect performance.
-  config.server_bundle_js_file = "server-bundle.js"
+  config.server_bundle_js_file = "app/assets/builds/server-bundle.js"
 
   # React on Rails 16 compatibility: Workaround for removed error handling
   #

config/webpack/serverWebpackConfig.js

@@ -45,12 +45,13 @@ const configureServer = () => {
 
   // Custom output for the server-bundle that matches the config in
   // config/initializers/react_on_rails.rb
+  const path = require('path');
   serverWebpackConfig.output = {
     filename: 'server-bundle.js',
     globalObject: 'this',
     // If using the React on Rails Pro node server renderer, uncomment the next line
     // libraryTarget: 'commonjs2',
-    path: config.outputPath,
+    path: path.resolve(__dirname, '../../app/assets/builds'),
     publicPath: config.publicPath,
     // https://webpack.js.org/configuration/output/#outputglobalobject
   };