refactor(app): extract previewChords to song-utils, simplify API_BASE, fix useEffect deps and cleanup

app/react-router.config.ts

@@ -0,0 +1,7 @@
+import type { Config } from "@react-router/dev/config";
+
+export default {
+  // Config options...
+  // Server-side render by default, to enable SPA mode set this to `false`
+  ssr: true,
+} satisfies Config;