Use config/swc.config.js instead of .swcrc for config merging

The .swcrc file completely overrides Shakapacker's swc-loader default
config, which includes important settings like loose: true and env config
for polyfills.

By using config/swc.config.js instead, our React-specific config gets
merged with Shakapacker's defaults via webpack-merge, preserving all the
necessary transpilation settings that Stimulus controllers need.

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

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

Author: justin808

.swcrc

@@ -0,0 +1,9 @@
+module.exports = {
+  jsc: {
+    transform: {
+      react: {
+        runtime: "automatic"
+      }
+    }
+  }
+};