[Feature] Drag-and-drop tabs with auto-split on drop

[RA1NCS]

Feature summary

Tabs in Athas currently can't be dragged or reordered, and there's no way to split the editor by dragging a tab onto another tab or to the edge of the editor area. This is a standard interaction model in VS Code, Zed, JetBrains IDEs, and most modern editors.

Requested behavior

• Tabs should be drag-and-droppable within the tab bar to reorder them
• Dragging a tab over another tab should offer a split (show a drop indicator for left/right/top/bottom split zones)
• Dragging a tab to the far right of the tab bar (or off the bar entirely) should trigger an auto-split, opening the tab in a new pane alongside the current layout
• Drop zones should show a visual highlight to indicate where the tab will land

Why this matters

This is a core editor workflow. Without it, organizing a multi-file workspace requires multiple manual steps that interrupt focus. Drag-to-split in particular is how most developers instinctively reach for when they want to compare two files side by side.

Implementation note

Since Athas uses React for the frontend, a library like @dnd-kit/core or react-dnd could handle the drag primitives. The split pane state would need to be updated accordingly to reflect the new layout tree on drop.