Service worker: network-first for the app shell

Cache-first meant players kept loading the previous build until their
second visit - a redeploy was invisible. The shell is now fetched from
the network first and falls back to the cache when offline; the icons and
manifest stay cache-first. Cache name bumped to v2 to drop old copies.

sw.js

@@ -1,7 +1,10 @@
 /* Mineflappy offline cache.
-   Cache-first for the app shell, with a background refresh so a new
+
-   deploy is picked up on the next visit. */
+   The app shell (index.html) is fetched NETWORK-FIRST so a new deploy
-const CACHE = 'mineflappy-v1';
+   reaches the player immediately; if the network is down it falls back to
+   the cached copy. The icons and manifest are cache-first because they
+   only change with a version bump. */
+const CACHE = 'mineflappy-v2';
 const ASSETS = [
   './',
   './index.html',
@@ -28,10 +31,33 @@ self.addEventListener('activate', e => {
   );
 });
 
+function isAppShell(url) {
+  return url.pathname.endsWith('/') ||
+         url.pathname.endsWith('/index.html') ||
+         url.pathname.endsWith('/sw.js');
+}
+
 self.addEventListener('fetch', e => {
   const req = e.request;
-  if (req.method !== 'GET' || new URL(req.url).origin !== location.origin) return;
+  if (req.method !== 'GET') return;
+  const url = new URL(req.url);
+  if (url.origin !== location.origin) return;
 
+  if (isAppShell(url)) {
+    /* network first - a redeploy shows up straight away */
+    e.respondWith(
+      fetch(req).then(res => {
+        if (res && res.status === 200) {
+          const copy = res.clone();
+          caches.open(CACHE).then(c => c.put(req, copy));
+        }
+        return res;
+      }).catch(() => caches.match(req).then(hit => hit || caches.match('./index.html')))
+    );
+    return;
+  }
+
+  /* everything else: cache first, refresh in the background */
   e.respondWith(
     caches.match(req).then(hit => {
       const network = fetch(req).then(res => {
@@ -40,7 +66,7 @@ self.addEventListener('fetch', e => {
           caches.open(CACHE).then(c => c.put(req, copy));
         }
         return res;
-      }).catch(() => hit || caches.match('./index.html'));
+      }).catch(() => hit);
       return hit || network;
     })
   );