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>

Gemfile.lock

@@ -13,7 +13,7 @@ GIT
 
 GIT
   remote: https://github.com/hotwired/turbo-rails.git
-  revision: 49be3f2d43edeeedad48cb492b35242fdb27af74
+  revision: 45a2a40be0a677808a3a269311f1b5abaa3b30ba
   branch: offline-cache
   specs:
     turbo-rails (2.0.21)