style: Improve markdown editor styling and functionality

- Update dark mode button icon and styling
- Add styling for new collection button
- Apply default iframe styles in preview pane
- Adjust vertical divider height in header buttons
- Improve handling of JSX-like attributes in markdown
- Add support for new collection functionality
- Refine file loading logic in view mode
- Improve dark mode toggle icon and integration
- Update UI for edit/view mode toggle button

static/css/file-tree.css

@@ -240,4 +240,14 @@ body.dark-mode .tree-empty-message {
 
 .sidebar::-webkit-scrollbar-thumb:hover {
     background-color: var(--text-secondary);
+}
+
+.new-collection-btn {
+    padding: 0.375rem 0.75rem;
+    font-size: 1rem;
+    border-radius: 0.25rem;
+    transition: all 0.2s ease;
+    color: var(--text-primary);
+    border: 1px solid var(--border-color);
+    background-color: transparent;
 }