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

static/css/layout.css

@@ -119,7 +119,7 @@ body {
 #sidebarPane {
     flex: 0 0 20%;
     min-width: 150px;
-    max-width: 40%;
+    max-width: 20%;
     padding: 0;
     height: 100%;
     overflow: hidden;