fix: repair service-worker.js duplicate install listener causing script eval failure

apply_patch previously duplicated the install event listener, causing a
ServiceWorker script evaluation error on load. Deduplicate and hoist
precacheShell() before the install listener.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/public/service-worker.js

@@ -142,8 +142,6 @@ async function replayQueuedRequests() {
   }
 }
 
-self.addEventListener('install', (event) => {
-self.addEventListener('install', (event) => {
 async function precacheShell() {
   const cache = await caches.open(CACHE_NAME);
   await Promise.all(