webapp: fix a11y contrast, heading hierarchy, prefix-safe modal actions

- Accessibility: dark ink (#00003d) on warning/success buttons, badges, and success alert - white text on #ff9500/#0ad64f failed WCAG AA (~2:1); dark-on-bright now passes.
- Heading hierarchy: sidebar brand h1 -> div.brand-title (with matching CSS selector), and standardized all page-header titles to a single <h1> per page (8 templates were h2).
- Prefix-safe modal JS: dashboard/backups/reports/enrollment delete+clone actions now build their form action from url_for(...'__K__'/'__F__').replace(...) instead of hardcoded paths, so they survive a URL-prefix mount.

webapp/templates/backups.html

@@ -3,7 +3,7 @@
 
 {% block content %}
 <div class="page-header">
-  <h2>Clonezilla Backups</h2>
+  <h1>Clonezilla Backups</h1>
   <div class="header-actions">
     <button class="pxe-btn pxe-btn-primary" data-bs-toggle="modal" data-bs-target="#uploadModal">
       <i class="bi bi-upload"></i> Upload Backup
@@ -134,7 +134,7 @@ document.getElementById('deleteModal').addEventListener('show.bs.modal', functio
   var filename = btn.getAttribute('data-filename');
   var machine = btn.getAttribute('data-machine');
   document.getElementById('deleteMachine').textContent = machine;
-  document.getElementById('deleteForm').action = '/backups/delete/' + encodeURIComponent(filename);
+  document.getElementById('deleteForm').action = "{{ url_for('clonezilla_delete', filename='__F__') }}".replace('__F__', encodeURIComponent(filename));
 });
 </script>
 {% endblock %}