chore: import upstream snapshot with attribution

api/server/utils/fallback.js

@@ -0,0 +1,20 @@
+/** Static asset extensions that must 404 when missing — serving the SPA's
+ * index.html for them breaks strict MIME checks and poisons SW/browser caches. */
+const STATIC_ASSET_EXT =
+  /\.(?:js|mjs|css|map|json|wasm|webmanifest|png|jpe?g|gif|svg|ico|webp|avif|woff2?|ttf|otf|eot)$/i;
+
+/**
+ * Creates the SPA fallback middleware: serves index.html for unmatched
+ * routes while returning 404 for missing static assets.
+ * @param {(req: import('express').Request, res: import('express').Response) => void} sendIndexHtml
+ */
+function createSpaFallback(sendIndexHtml) {
+  return (req, res) => {
+    if (STATIC_ASSET_EXT.test(req.path)) {
+      return res.status(404).end();
+    }
+    return sendIndexHtml(req, res);
+  };
+}
+
+module.exports = createSpaFallback;