fix: switch SW to network-first strategy so PWA always gets fresh updates

Cache-first meant users were stuck on old versions until manual cache clear.
Network-first fetches live, updates cache, falls back to cache when offline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'ns-v20';
+const CACHE = 'ns-v21';
 const PRECACHE = ['/', 'index.html', 'styles.css', 'app.js', 'sw.js', 'manifest.json', 'icon.svg'];
 const SKIP_URLS = ['/rest/stream', '/rest/download', '/config.js'];
 
@@ -19,14 +19,14 @@ self.addEventListener('activate', e => {
 self.addEventListener('fetch', e => {
   if (SKIP_URLS.some(u => e.request.url.includes(u))) return;
   e.respondWith(
-    caches.match(e.request).then(cached =>
-      cached || fetch(e.request).then(res => {
+    fetch(e.request)
+      .then(res => {
         if (res.status === 200 && e.request.method === 'GET') {
           const clone = res.clone();
           caches.open(CACHE).then(c => c.put(e.request, clone));
         }
         return res;
-      }).catch(() => cached)
-    )
+      })
+      .catch(() => caches.match(e.request).then(r => r || Response.error()))
   );
 });