feat: cross-container tab dragging in split-pane layout

[aterrylu]

Summary

• Enable dragging tabs between leaf containers in the split-pane layout
• Tabs can be dropped on another container's center (add as tab) or edges (create new split)
• When the source container's last tab is dragged away, the container collapses automatically
• Works for all tab types: session tabs, preview panes, org chart, templates

How it works

The implementation is minimal (single file change) because it leverages the existing drag infrastructure:

graph LR
    A[TabBar<br/>onDragStart] -->|startDrag| B[DragContext<br/>isDragging=true]
    B --> C[SessionMountLayer<br/>pointerEvents=none]
    B --> D[DropZoneOverlay<br/>shows drop zones]
    D -->|existing pane| E[movePaneToLeaf<br/>removes from source<br/>adds to target]
    D -->|new pane| F[splitLeafWithPane<br/>or addTabToLeaf]

Loading

The same DragContext → DropZoneOverlay → movePaneToLeaf pipeline used by sidebar drag now also handles tab drag. No new components or store actions needed.

Changes

• packages/dashboard/src/layout/TabBar.tsx — Add draggable, onDragStart, onDragEnd handlers to tab elements. Wire into existing DragContext. Visual feedback: dimmed opacity on dragged tab, grab cursor.

Test plan

• Open two panes in a split layout
• Drag a tab from one container to the other (center = add as tab)
• Drag a tab to the edge of another container (creates new split)
• Drag the last tab from a container (container should collapse)
• Open a markdown preview (ctrl-click), drag it to the other side for side-by-side view
• Verify no terminal flickering or WebSocket drops during drag
• Verify same-leaf drag shows no drop zones (correct suppression)

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