Fix aria-hidden accessibility warning on modal

Removed static aria-hidden attribute from modal element to fix
"Blocked aria-hidden on an element because its descendant retained
focus" accessibility warning.

Changes:
- Remove aria-hidden="true" from modal root element
- Add role="document" to modal-dialog for better accessibility
- Let Bootstrap 5 manage aria-hidden dynamically on open/close

The static aria-hidden="true" was conflicting with focus management
when the modal opened. Bootstrap 5 handles this attribute dynamically,
so it should not be set in the HTML.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/views/layouts/shared/_header.html.erb

@@ -113,8 +113,8 @@
   </header>
 
   <!-- Credentials Modal -->
-  <div class="modal fade" id="credentialsModal" tabindex="-1" aria-labelledby="credentialsModalLabel" aria-hidden="true">
-    <div class="modal-dialog">
+  <div class="modal fade" id="credentialsModal" tabindex="-1" aria-labelledby="credentialsModalLabel">
+    <div class="modal-dialog" role="document">
       <div class="modal-content">
         <div class="modal-header">
           <h5 class="modal-title" id="credentialsModalLabel">