Fix CSS modules config for server bundle

Preserve namedExport and exportLocalsConvention settings when adding
exportOnlyLocals for SSR. This prevents CSS module imports from being
undefined in the server-rendered bundle.

The previous code was replacing the entire modules object, which broke
the namedExport configuration required by Shakapacker 9.

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

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

Author: justin808

config/rspack/serverRspackConfig.js

@@ -92,8 +92,12 @@ const configureServer = () => {
 
         return testValue.includes('css-loader');
       });
-      if (cssLoader && cssLoader.options) {
-        cssLoader.options.modules = { exportOnlyLocals: true };
+      if (cssLoader && cssLoader.options && cssLoader.options.modules) {
+        // Preserve existing modules config but add exportOnlyLocals for SSR
+        cssLoader.options.modules = {
+          ...cssLoader.options.modules,
+          exportOnlyLocals: true,
+        };
       }
 
       // Skip writing image files during SSR by setting emitFile to false