Make the editor and preview panes resizable

### Feature request

It would be great if the editor and preview panes could be resized by dragging the divider between them.

### Problem

Currently, the editor and preview appear to have a fixed width. Depending on the task, I sometimes need more space for the rendered preview and other times more space for the Markdown source.

### Proposed solution

Allow users to drag the divider between the editor and preview panes to adjust their widths.

Some possible behaviors:

- Drag the divider horizontally to resize the panes.
- Remember the selected layout between sessions (optional).
- Keep a reasonable minimum width for both panes.

### Benefits

- More comfortable editing experience.
- Better readability for complex rendered content.
- Allows users to customize the workspace based on their workflow.

Thank you for this great project!

Locate the editor and preview layout entry points and inspect how the divider between them is rendered. Start by tracing the pane sizing behavior, then verify that dragging adjusts both panes and that each retains a reasonable minimum width; persistence between sessions is listed as optional.

Written by the indexing model from the issue text.