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" })
+  }
+}

app/views/my/menus/_settings.html.erb

@@ -6,7 +6,7 @@
 
   <%= tag.li class: "popup__item", data: { filter_target: "item", navigable_list_target: "item" } do %>
     <%= icon_tag "logout", class: "popup__icon" %>
-    <%= button_to session_path(script_name: nil), method: :delete, class: "popup__btn btn", data: { turbo: false } do %>
+    <%= button_to session_path(script_name: nil), method: :delete, class: "popup__btn btn", form: { data: { turbo: false, controller: "logout", action: "submit->logout#clearCache" } } do %>
       <span>Sign out</span>
     <% end %>
   <% end %>

app/views/users/show.html.erb

@@ -41,7 +41,7 @@
       <% if Current.user == @user %>
         <hr class="separator--horizontal full-width flex-item-grow margin-block-start-double" style="--border-color: var(--color-ink-light);" aria-hidden="true">
 
-        <%= button_to session_path(script_name: nil), method: :delete, class: "btn txt-x-small center", data: { turbo: false } do %>
+        <%= button_to session_path(script_name: nil), method: :delete, class: "btn txt-x-small center", form: { data: { turbo: false, controller: "logout", action: "submit->logout#clearCache" } } do %>
           <span>Sign out of Fizzy on this device</span>
         <% end %>
       <% end %>