Snap Layouts: drag notes to editor edges for split view

[M0-kii]

Goal:
Let users drag a note from the sidebar and drop it on an editor edge to create a split-pane view, similar to Windows 11 Snap Layouts.

Implementation Notes:

• When dragging a note over the editor area, show snap zones (left half, right half, top half, bottom half, full)
• Dropping on a zone opens the note in that section of the split pane
• Use existing @dnd-kit infrastructure with custom drop targets overlaid on the editor
• Highlight the active snap zone with a translucent overlay during drag
• Support snapping back out (closing a split) by dragging the note back to the sidebar

Constraints:

• Must not interfere with existing drag-to-reorder in the sidebar
• Must work with the existing SplitDivider and split-pane layout
• Keep the 5px activation distance for PointerSensor to avoid accidental triggers

Labels: enhancement