Clear offline cache on sign-in as well as sign-out

Rename logout controller to clear-offline-cache and attach it to the
magic link verification form so the service worker cache is cleared
when a different user signs in.

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

app/javascript/controllers/clear_offline_cache_controller.js

@@ -0,0 +1,8 @@
+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  async clearCache() {
+    const registration = await navigator.serviceWorker?.ready
+    registration?.active?.postMessage({ action: "clearCache" })
+  }
+}