feat: Implement collection deletion and loading spinners

- Add API endpoint and handler to delete collections
- Introduce LoadingSpinner component for async operations
- Show loading spinners during file loading and preview rendering
- Enhance modal accessibility by removing aria-hidden attribute
- Refactor delete functionality to distinguish between collections and files/folders
- Remove unused collection definitions from config

templates/index.html

@@ -238,6 +238,7 @@
     <script src="/static/js/sidebar-toggle.js" defer></script>
     <script src="/static/js/collection-selector.js" defer></script>
     <script src="/static/js/editor-drop-handler.js" defer></script>
+    <script src="/static/js/loading-spinner.js" defer></script>
 
     <!-- Core Application Modules -->
     <script src="/static/js/webdav-client.js" defer></script>