Only start offline mode when signed in

Gate Turbo.offline.start() behind Current.user so the service worker
is only registered for authenticated users. This avoids errors on
unauthenticated pages where /service-worker.js requires auth.

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

app/javascript/initializers/offline.js

@@ -1,7 +1,9 @@
 import { Turbo } from "@hotwired/turbo-rails"
 
-Turbo.offline.start("/service-worker.js", {
-  scope: "/",
-  native: true,
-  preload: /\/assets\//
-})
+if (Current.user) {
+  Turbo.offline.start("/service-worker.js", {
+    scope: "/",
+    native: true,
+    preload: /\/assets\//
+  })
+}