Add interactive task lists and table insertion to markdown editor

- Enable task list checkbox toggling in preview mode with live content updates
- Add task list and table insertion buttons to InsertToolbar
- Implement smart block snippet insertion with automatic newline handling
- Add horizontal scroll wrapper for wide tables in preview
- Fix editor scroll position preservation during content updates
- Use useLayoutEffect to prevent scroll jumps when textarea auto-resizes
- Update task list styling

src/index.css

@@ -232,10 +232,20 @@ code {
   height: 1.25rem;
   margin-right: 0.75rem;
   margin-top: 0.32rem;
-  cursor: default;
+  cursor: pointer;
   flex-shrink: 0;
 }
 
 .prose input[type="checkbox"]:checked {
   accent-color: #16a34a;
 }
+
+.prose .markdown-table-wrapper {
+  overflow-x: auto;
+  margin: 1.5em 0;
+}
+
+.prose .markdown-table-wrapper table {
+  margin: 0;
+  min-width: 100%;
+}