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/js/app.js

@@ -337,6 +337,12 @@ document.addEventListener('DOMContentLoaded', async () => {
 
                 Logger.info(`Previewing binary file: ${item.path}`);
 
+                // Initialize and show loading spinner for binary file preview
+                editor.initLoadingSpinners();
+                if (editor.previewSpinner) {
+                    editor.previewSpinner.show(`Loading ${fileType.toLowerCase()}...`);
+                }
+
                 // Set flag to prevent auto-update of preview
                 editor.isShowingCustomPreview = true;
 
@@ -403,6 +409,14 @@ document.addEventListener('DOMContentLoaded', async () => {
                 // Display in preview pane
                 editor.previewElement.innerHTML = previewHtml;
 
+                // Hide loading spinner after content is set
+                // Add small delay for images to start loading
+                setTimeout(() => {
+                    if (editor.previewSpinner) {
+                        editor.previewSpinner.hide();
+                    }
+                }, fileType === 'Image' ? 300 : 100);
+
                 // Highlight the file in the tree
                 fileTree.selectAndExpandPath(item.path);