Clear offline cache when logging out

Adds a logout Stimulus controller that sends a message to the service
worker to clear all cached content when the user logs out. This ensures
that cached data from one user isn't accessible after logout.

The implementation:
- Adds logout_controller.js that posts { action: "clearCache" } to the
  service worker via postMessage
- Updates logout buttons to use the controller on form submission

Also fixes data-turbo placement: moved from button to form element where
it actually takes effect for disabling Turbo Drive form submissions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/javascript/controllers/logout_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" })
+  }
+}