Add loading spinner for Turbo Frame updates

Why these changes are being introduced:

Users experience long delays when switching between search tabs due to
slow Primo API responses, with no visual feedback indicating that the
system is processing their request.

Relevant ticket(s):

- [USE-134](https://mitlibraries.atlassian.net/browse/USE-134)

How this addresses that need:

This adds a CSS spinner animation that appears during Turbo Frame
updates. It uses Turbo's built-in `busy` attribute for reliable state
detection. Turbo also adds `aria-busy` for screen reader users.

Side effects of this change:

- Adds data-turbo-action="advance" to search results frame.
- Spinner may briefly show during fast API responses.
- Deprecated Turbo API syntax has been updated to current syntax.
- This is different the from built-in Turbo progress bar that we use for
page reloads, as that feature is not available for Turbo Frame updates.
It's fairly straightforward to rebuild in JS if we decide we want
consistent loading progress indicators.

Author: jazairi

app/assets/stylesheets/partials/_results.scss

@@ -119,3 +119,36 @@
     line-height: 1.1;
   }
 }
+
+// Loading indicator for turbo frame updates
+// https://discuss.hotwired.dev/t/show-spinner-everytime-async-frame-reloads/3483/3
+@keyframes spinner {
+  to { 
+    transform: rotate(360deg); 
+  }
+}
+
+[busy]:not([no-spinner]) {
+  position: relative;
+
+  > * {
+    opacity: 0.5;
+  }
+
+  &::after {
+    content: '';
+    box-sizing: border-box;
+    position: fixed;
+    top: 50%;
+    left: 50%;
+    width: 2.5rem;
+    height: 2.5rem;
+    margin-top: -1.25rem;
+    margin-left: -1.25rem;
+    border-radius: 50%;
+    border: 0.275rem solid rgba($color-red-500, 0.3);
+    border-top-color: $color-red-500;
+    animation: spinner 0.6s linear infinite;
+    z-index: 9999;
+  }
+}

app/javascript/application.js

@@ -3,4 +3,4 @@ import "@hotwired/turbo-rails"
 import "controllers"
 
 // Show the progress bar after 200 milliseconds, not the default 500
-window.Turbo.setProgressBarDelay(200);
+Turbo.config.drive.progressBarDelay = 200;

app/views/search/results.html.erb

@@ -14,7 +14,7 @@
 
   <%= render(partial: 'trigger_tacos') if tacos_enabled? %>
 
-  <%= turbo_frame_tag "search-results" do %>
+  <%= turbo_frame_tag "search-results", data: { turbo_action: "advance" } do %>
     <div class="layout-3q1q layout-band top-space">
       <main id="results" class="col3q wrap-results">
         <% if @show_primo_continuation %>