Treat 5xx navigation responses as offline in the service worker

fetch() only rejects on true network failures, not bad status codes,
so a reverse proxy (Nginx Proxy Manager) returning 502/503 while the
app server itself is down was passed through as-is instead of falling
back to the offline experience. Now a >=500 response is treated the
same as a network error.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

templates/sw.js

@@ -38,9 +38,18 @@ self.addEventListener('fetch', (event) => {
     // Navigations: try the network first, fall back to the offline page.
     // The dashboard route falls back to the offline-capable task app instead
     // of the generic offline page, since it can still show/edit cached tasks.
+    // A 5xx response (e.g. a reverse proxy up-front returning 502/503 while
+    // the app server itself is down) is treated the same as a network
+    // failure - fetch() only rejects on true network errors, not bad status
+    // codes, so that has to be checked explicitly.
     if (request.mode === 'navigate') {
         event.respondWith(
-            fetch(request).catch(() => {
+            fetch(request).then((response) => {
+                if (response.status >= 500) {
+                    throw new Error(`Server error: ${response.status}`);
+                }
+                return response;
+            }).catch(() => {
                 const path = new URL(request.url).pathname;
                 return caches.match(path === '/' ? OFFLINE_TASKS_URL : OFFLINE_URL);
             })