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/layout.css

@@ -232,6 +232,14 @@ body {
     margin-top: 0;
 }
 
+/* Iframe styles in preview - minimal defaults that can be overridden */
+#preview iframe {
+    border: none;
+    /* Default to no border, can be overridden by inline styles */
+    display: block;
+    /* Prevent inline spacing issues */
+}
+
 /* View Mode Styles */
 body.view-mode #editorPane {
     display: none;