Use Babel for JavaScript files, SWC for TypeScript only

Add Babel loader for .js/.jsx files to fix Stimulus compatibility issues.
SWC's class transformation breaks Stimulus event listener binding, so we
use Babel for JavaScript (including Stimulus controllers) while keeping
SWC for TypeScript files to maintain performance benefits.

This hybrid approach matches the reference implementation and resolves
the RSpec test failures with Stimulus controllers.

Author: justin808

config/webpack/commonWebpackConfig.js

@@ -69,7 +69,47 @@ if (scssConfigIndex === -1) {
 }
 
 // Copy the object using merge b/c the baseClientWebpackConfig and commonOptions are mutable globals
-const commonWebpackConfig = () => merge({}, baseClientWebpackConfig, commonOptions, ignoreWarningsConfig);
+const commonWebpackConfig = () => {
+  const config = merge({}, baseClientWebpackConfig, commonOptions, ignoreWarningsConfig);
+
+  // Find the SWC rule and get its include/exclude
+  const swcRuleIndex = config.module.rules.findIndex(rule =>
+    rule.test && /\.(ts|tsx|js|jsx|mjs|coffee)/.test(rule.test.toString())
+  );
+
+  if (swcRuleIndex !== -1) {
+    const swcRule = config.module.rules[swcRuleIndex];
+
+    // Insert Babel rule BEFORE SWC rule so it matches .js files first
+    // This allows Babel to handle all JavaScript while SWC continues to match TypeScript
+    config.module.rules.splice(swcRuleIndex, 0, {
+      test: /\.(js|jsx|mjs)(\.erb)?$/,
+      include: swcRule.include,
+      exclude: swcRule.exclude,
+      use: {
+        loader: 'babel-loader',
+        options: {
+          presets: [
+            ['@babel/preset-env', {
+              useBuiltIns: 'entry',
+              corejs: 3,
+              modules: 'auto',
+              bugfixes: true,
+              exclude: ['transform-typeof-symbol']
+            }],
+            ['@babel/preset-react', {
+              runtime: 'automatic',
+              development: process.env.NODE_ENV !== 'production',
+              useBuiltIns: true
+            }]
+          ]
+        }
+      }
+    });
+  }
+
+  return config;
+};
 
 module.exports = commonWebpackConfig;