Fix CSS modules fallback with identity proxy loader

The previous fallback wasn't providing the named exports that CSS modules
require. This fix uses an inline data URI loader to provide a Proxy object
that returns any property name as a string, allowing CSS module imports
like `import * as styles from './file.module.css'` to work in test
environments where CSS loaders aren't available.

The proxy returns the property name as-is for named imports (e.g.,
`styles.bright` returns "bright") and returns itself for default imports,
making it work with both import patterns.

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

Co-Authored-By: Claude <[email protected]>

Author: justin808

lib/generators/react_on_rails/templates/base/base/config/webpack/commonWebpackConfig.js.tt

@@ -44,15 +44,13 @@ if (cssLoaderAvailable && MiniCssExtractPlugin) {
     ],
   };
 } else {
-  // Fallback: ignore CSS files when loaders aren't available (for testing)
+  // Fallback: provide identity object for CSS modules (for testing)
   commonOptions.module = {
     rules: [
       {
         test: /\.module\.css$/i,
-        type: 'asset/resource',
-        generator: {
-          emit: false,
-        },
+        type: 'javascript/auto',
+        loader: 'data:text/javascript,module.exports=new Proxy({},{get:function(target,prop){return prop==="default"?this:prop}})',
       },
     ],
   };