Show the update banner only when the running build differs from the server and make Refresh UI land the new shell on flaky links

frontend/sw.js

@@ -11,12 +11,12 @@
  * Auto-update flow:
  *   1. New SW installs alongside the old one and waits (skipWaiting() is NOT
  *      called automatically — see the install handler below).
- *   2. Client (app.js registerServiceWorker()) notices the waiting worker
- *      and shows the "Update ready" banner.
- *   3. User clicks "Reload now" → client (applyUpdate() in sw-update.js)
- *      posts SKIP_WAITING to the waiting worker and waits for it to take
- *      control (controllerchange) before reloading — it does NOT unregister
- *      or wipe caches, so the reload doesn't force a brand-new install.
+ *   2. The client (app.js) shows the "Update available" banner only when the
+ *      build it is running differs from the server's /api/version buildTag —
+ *      a waiting worker or a broadcast is merely a prompt to re-check.
+ *   3. "Refresh UI" (sw-update.js) re-downloads every shell file into every
+ *      versioned shell cache (all-or-nothing), activates a waiting worker if
+ *      there is one (SKIP_WAITING), and reloads once.
  *   4. activate: if an older *versioned shell cache* is found (i.e. this
  *      activation is genuinely replacing a previous deploy, not just the
  *      first-ever install of a freshly (re)registered worker), broadcast
@@ -68,14 +68,40 @@ const SHELL = [
 ];
 
 // ---- Install: pre-cache the app shell ----
+// skipWaiting() is NOT called here — the page activates a waiting worker
+// from "Refresh UI" (see sw-update.js).
 self.addEventListener('install', (e) => {
-  e.waitUntil(
-    caches.open(CACHE).then((cache) => cache.addAll(SHELL))
-    // skipWaiting() is NOT called here — we wait for the client to confirm
-    // before activating, so the update banner can appear first.
-  );
+  e.waitUntil(precacheShell());
 });
 
+// Fresh requests (`cache: 'reload'` skips the browser HTTP cache, which could
+// otherwise hand the new worker yesterday's app.js), a few attempts per file
+// because the homelab link drops requests, and a failed install deletes its
+// partial cache — a leftover ytplayer-<tag> cache is exactly what activate
+// uses to decide "this is a genuine update", and it used to re-open the
+// update banner on pages that were already current.
+const PRECACHE_ATTEMPTS = 3;
+async function precacheShell() {
+  const cache = await caches.open(CACHE);
+  try {
+    await Promise.all(SHELL.map(async (url) => {
+      let lastErr = null;
+      for (let attempt = 0; attempt < PRECACHE_ATTEMPTS; attempt++) {
+        try {
+          const res = await fetch(new Request(url, { cache: 'reload' }));
+          if (!res.ok) throw new Error(url + ' → ' + res.status);
+          await cache.put(url, res);
+          return;
+        } catch (err) { lastErr = err; }
+      }
+      throw lastErr;
+    }));
+  } catch (err) {
+    await caches.delete(CACHE);
+    throw err;
+  }
+}
+
 // ---- Activate: evict old caches, claim clients, notify about update ----
 self.addEventListener('activate', (e) => {
   e.waitUntil((async () => {