fix(offline): fall back to cached shell on upstream 5xx

Service worker previously only fell back to cache when fetch threw
(network unreachable). A 502/503/504 from the origin returned
successfully with !response.ok, so the bad page was passed through to
the user. Now upstream 5xx is treated like a network failure: try
cached page, then offline-shell redirect for recipe routes, then the
styled offline page. 4xx still passes through unchanged.

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "homepage",
-	"version": "1.67.3",
+	"version": "1.67.4",
 	"private": true,
 	"type": "module",
 	"scripts": {