Update webpack config templates for unified rspack/webpack support

Implements unified configuration approach where the same webpack config
files work with both webpack and rspack bundlers, based on the
assets_bundler setting in shakapacker.yml.

## Changes

**development.js.tt**:
- Add config import from shakapacker to access assets_bundler setting
- Conditional React Refresh plugin loading:
  - Rspack: @rspack/plugin-react-refresh
  - Webpack: @pmmmwh/react-refresh-webpack-plugin
- Prevents "window not found" errors when using rspack

**serverWebpackConfig.js.tt**:
- Replace hardcoded webpack require with bundler variable
- Bundler conditionally requires @rspack/core or webpack
- Use bundler.optimize.LimitChunkCountPlugin instead of webpack-specific
- Eliminates warnings about webpack when using rspack

**RSPACK_IMPLEMENTATION.md**:
- Document webpack config template changes
- Explain unified configuration approach

## Benefits

- Single set of config files works for both bundlers
- No warnings when using rspack about webpack.config.js
- Seamless switching between bundlers via bin/switch-bundler
- Follows pattern from react_on_rails-demos PR #20

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

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

Author: justin808

RSPACK_IMPLEMENTATION.md

@@ -28,7 +28,26 @@ The `--rspack` flag allows users to generate a React on Rails application using
   - Adds `assets_bundler: 'rspack'` to shakapacker.yml default section
   - Changes `webpack_loader` to `'swc'` (Rspack works best with SWC transpiler)
 
-### 3. Bundler Switching Script (`lib/generators/react_on_rails/templates/base/base/bin/switch-bundler`)
+### 3. Webpack Configuration Templates
+
+Updated webpack configuration templates to support both webpack and rspack bundlers with unified config approach:
+
+**development.js.tt**:
+
+- Added `config` to shakapacker require to access `assets_bundler` setting
+- Conditional React Refresh plugin loading based on `config.assets_bundler`:
+  - Rspack: Uses `@rspack/plugin-react-refresh`
+  - Webpack: Uses `@pmmmwh/react-refresh-webpack-plugin`
+- Prevents "window not found" errors when using rspack
+
+**serverWebpackConfig.js.tt**:
+
+- Added `bundler` variable that conditionally requires `@rspack/core` or `webpack`
+- Changed `webpack.optimize.LimitChunkCountPlugin` to `bundler.optimize.LimitChunkCountPlugin`
+- Enables same config to work with both bundlers without warnings
+- Avoids hardcoding webpack-specific imports
+
+### 4. Bundler Switching Script (`lib/generators/react_on_rails/templates/base/base/bin/switch-bundler`)
 
 Created a new executable script that allows switching between webpack and rspack after installation:
 

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

@@ -1,20 +1,26 @@
 <%= add_documentation_reference(config[:message], "// https://github.com/shakacode/react_on_rails_demo_ssr_hmr/blob/master/config/webpack/development.js") %>
 
-const { devServer, inliningCss } = require('shakapacker');
+const { devServer, inliningCss, config } = require('shakapacker');
 
 const generateWebpackConfigs = require('./generateWebpackConfigs');
 
 const developmentEnvOnly = (clientWebpackConfig, _serverWebpackConfig) => {
-  // React Refresh (Fast Refresh) setup - only when webpack-dev-server is running (HMR mode)
-  // This matches the condition in generateWebpackConfigs.js and babel.config.js
+  // React Refresh (Fast Refresh) setup - only when dev server is running (HMR mode)
   if (process.env.WEBPACK_SERVE) {
     // eslint-disable-next-line global-require
-    const ReactRefreshWebpackPlugin = require('@pmmmwh/react-refresh-webpack-plugin');
-    clientWebpackConfig.plugins.push(
-      new ReactRefreshWebpackPlugin({
-        // Use default overlay configuration for better compatibility
-      }),
-    );
+    if (config.assets_bundler === 'rspack') {
+      // Rspack uses @rspack/plugin-react-refresh for React Fast Refresh
+      const ReactRefreshPlugin = require('@rspack/plugin-react-refresh');
+      clientWebpackConfig.plugins.push(new ReactRefreshPlugin());
+    } else {
+      // Webpack uses @pmmmwh/react-refresh-webpack-plugin
+      const ReactRefreshWebpackPlugin = require('@pmmmwh/react-refresh-webpack-plugin');
+      clientWebpackConfig.plugins.push(
+        new ReactRefreshWebpackPlugin({
+          // Use default overlay configuration for better compatibility
+        }),
+      );
+    }
   }
 };
 

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

@@ -3,7 +3,9 @@
 const { merge, config } = require('shakapacker');
 const commonWebpackConfig = require('./commonWebpackConfig');
 
-const webpack = require('webpack');
+const bundler = config.assets_bundler === 'rspack'
+  ? require('@rspack/core')
+  : require('webpack');
 
 const configureServer = () => {
   // We need to use "merge" because the clientConfigObject, EVEN after running
@@ -40,7 +42,7 @@ const configureServer = () => {
   serverWebpackConfig.optimization = {
     minimize: false,
   };
-  serverWebpackConfig.plugins.unshift(new webpack.optimize.LimitChunkCountPlugin({ maxChunks: 1 }));
+  serverWebpackConfig.plugins.unshift(new bundler.optimize.LimitChunkCountPlugin({ maxChunks: 1 }));
 
   // Custom output for the server-bundle that matches the config in
   // config/initializers/react_on_rails.rb