Add a two-column split view.

Requirements

• Render minimal toolbar in upper-right corner that displays two icons: one for a single column and one for two columns (split view). Initially the single column icon will be white (enabled) and the two column will be gray (disabled). When one is tapped, it enables the respective mode, which is tracked in state and local storage.
• 50/50 split, left and right columns, each with their own hierarchy, with reasonable margin in between.
• Make all keyboard shortcuts and gestures affect the cursor only in the “active view”.
• Clicking or tapping a thought activates the respective view and sets the cursor.
• Cursor should be hidden in the inactive view.
• Fade in second column upon activation.
• When entering split view, default the left column to the cursor and default the right column to its last known cursor, or the same as the left column if it has yet to get a cursor.
• Able to drag and drop between views.
• Automatically enter split view on mobile when switching from portrait to landscape.
• Separate breadcrumb navigation centered over each column.
• “Single Column” link centered at the top of each column with reasonable margins should return the user to single the column view for the given column.
• Dark and Light Mode support.

Rough mockup (partial):