## Summary
Fix panic "cannot update workspace::pane::Pane while it is already being
updated" when dragging terminal tabs to split the pane.

## Problem
When dragging a terminal tab to create a split, the app panics due to
re-entrancy: the drop handler calls `terminal_panel.center.split()`
synchronously, which invokes `mark_positions()` that tries to update all
panes in the group. When the pane being updated is part of the terminal
panel's center group, this causes a re-entrancy panic.

## Solution
Defer the split operation using `cx.spawn_in()`, similar to how
`move_item` was already deferred in the same handler. This ensures the
split (and subsequent `mark_positions()` call) runs after the current
pane update completes.

## Test plan
- Open terminal panel
- Create a terminal tab
- Drag the terminal tab to split the pane
- Verify no panic occurs and split works correctly