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/file-tree-actions.js

@@ -189,26 +189,62 @@ class FileTreeActions {
         },
 
         delete: async function (path, isDir) {
-            const name = path.split('/').pop();
+            const name = path.split('/').pop() || this.webdavClient.currentCollection;
             const type = isDir ? 'folder' : 'file';
 
-            const confirmed = await window.ModalManager.confirm(
-                `Are you sure you want to delete ${name}?`,
-                `Delete this ${type}?`,
-                true
-            );
+            // Check if this is a root-level collection (empty path or single-level path)
+            const pathParts = path.split('/').filter(p => p.length > 0);
+            const isCollection = pathParts.length === 0;
 
-            if (!confirmed) return;
+            if (isCollection) {
+                // Deleting a collection - use backend API
+                const confirmed = await window.ModalManager.confirm(
+                    `Are you sure you want to delete the collection "${name}"? This will delete all files and folders in it.`,
+                    'Delete Collection?',
+                    true
+                );
 
-            await this.webdavClient.delete(path);
+                if (!confirmed) return;
 
-            // Clear undo history since manual delete occurred
-            if (this.fileTree.lastMoveOperation) {
-                this.fileTree.lastMoveOperation = null;
+                try {
+                    // Call backend API to delete collection
+                    const response = await fetch(`/api/collections/${name}`, {
+                        method: 'DELETE'
+                    });
+
+                    if (!response.ok) {
+                        const error = await response.text();
+                        throw new Error(error || 'Failed to delete collection');
+                    }
+
+                    showNotification(`Collection "${name}" deleted successfully`, 'success');
+
+                    // Reload the page to refresh collections list
+                    window.location.href = '/';
+                } catch (error) {
+                    Logger.error('Failed to delete collection:', error);
+                    showNotification(`Failed to delete collection: ${error.message}`, 'error');
+                }
+            } else {
+                // Deleting a regular file/folder - use WebDAV
+                const confirmed = await window.ModalManager.confirm(
+                    `Are you sure you want to delete ${name}?`,
+                    `Delete this ${type}?`,
+                    true
+                );
+
+                if (!confirmed) return;
+
+                await this.webdavClient.delete(path);
+
+                // Clear undo history since manual delete occurred
+                if (this.fileTree.lastMoveOperation) {
+                    this.fileTree.lastMoveOperation = null;
+                }
+
+                await this.fileTree.load();
+                showNotification(`Deleted ${name}`, 'success');
             }
-
-            await this.fileTree.load();
-            showNotification(`Deleted ${name}`, 'success');
         },
 
         download: async function (path, isDir) {