Fix modal not displaying by disposing stale instances

Fixed modal showing backdrop but not the modal itself by explicitly
disposing old instances and adding a timing delay.

Changes:
- Dispose of existing modal instance before creating new one
- Create fresh modal with explicit options (backdrop, keyboard, focus)
- Add 10ms setTimeout before show() to ensure DOM readiness
- Remove getOrCreateInstance which was causing conflicts

The modal was creating a backdrop but staying display:none because
getOrCreateInstance was returning a stale modal instance that couldn't
properly transition. Disposing and recreating fixes this.

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

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

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

@@ -147,15 +147,29 @@
             .then(html => {
               document.getElementById('modal_content').innerHTML = html;
 
-              // Use Bootstrap 5 Modal API directly
-              const modal = bootstrap.Modal.getOrCreateInstance(modalElement);
-              modal.show();
+              // Dispose of any existing modal instance first
+              const existingModal = bootstrap.Modal.getInstance(modalElement);
+              if (existingModal) {
+                existingModal.dispose();
+              }
+
+              // Create fresh modal instance and show
+              const modal = new bootstrap.Modal(modalElement, {
+                backdrop: true,
+                keyboard: true,
+                focus: true
+              });
+
+              // Force show after a small delay to ensure DOM is ready
+              setTimeout(() => {
+                modal.show();
+              }, 10);
             })
             .catch(error => {
               console.error('Error loading credentials:', error);
               document.getElementById('modal_content').innerHTML = '<p class="text-danger">Error loading credentials. Please try again.</p>';
 
-              const modal = bootstrap.Modal.getOrCreateInstance(modalElement);
+              const modal = new bootstrap.Modal(modalElement);
               modal.show();
             });
         });