fix: fall back on proxy error responses

apps/web/vite.config.ts

@@ -123,6 +123,7 @@ self.addEventListener("fetch", (event) => {
   if (event.request.mode === "navigate") {
     event.respondWith(
       fetch(event.request).then((response) => {
+        if (!response.ok) throw new Error("navigation unavailable");
         const copy = response.clone();
         void caches.open(CACHE_NAME).then((cache) => cache.put("/", copy));
         return response;