Fix stale UI cache with network-first app shell and forced SW updates

app/public/index.html

@@ -297,7 +297,18 @@
 <script src="app.js"></script>
 <script>
   if ('serviceWorker' in navigator) {
-    navigator.serviceWorker.register('/sw.js');
+    window.addEventListener('load', () => {
+      navigator.serviceWorker.register('/sw.js').then(reg => {
+        reg.update().catch(() => {});
+      });
+
+      let refreshing = false;
+      navigator.serviceWorker.addEventListener('controllerchange', () => {
+        if (refreshing) return;
+        refreshing = true;
+        window.location.reload();
+      });
+    });
   }
 </script>
 </body>

app/public/sw.js

@@ -1,6 +1,6 @@
 'use strict';
 
-const CACHE = 'weather-v2';
+const CACHE = 'weather-v3';
 
 // App shell — cached on install for offline use
 const PRECACHE = [
@@ -45,6 +45,28 @@ self.addEventListener('fetch', event => {
     return;
   }
 
+  const isAppShellRequest =
+    request.mode === 'navigate' ||
+    request.destination === 'document' ||
+    request.destination === 'script' ||
+    request.destination === 'style';
+
+  // Prefer fresh app shell assets so UI updates are visible immediately.
+  if (isAppShellRequest) {
+    event.respondWith(
+      fetch(request)
+        .then(response => {
+          if (response.ok) {
+            const clone = response.clone();
+            caches.open(CACHE).then(cache => cache.put(request, clone));
+          }
+          return response;
+        })
+        .catch(() => caches.match(request).then(cached => cached || caches.match('/')))
+    );
+    return;
+  }
+
   event.respondWith(
     caches.match(request).then(cached => {
       if (cached) return cached;