Drag a pane's toolbar onto another pane to swap them

New interaction: click-and-drag any pane's toolbar onto another pane
to swap their positions in the tree. The shells / scrollback stay
intact (each leaf keeps its data; only the tree slot it occupies
changes).

Implementation:
- tree.ts: `swapLeaves(root, idA, idB)` walks the tree once,
  substituting one leaf for the other at each occurrence. The leaf
  objects themselves carry their id/distro/cwd/label/broadcast across,
  so React preserves the LeafPane instances via the flat-list keying.

- orchestration.tsx: add drag lifecycle to the context —
  dragSourceId / dragOverId (reactive) plus beginHeaderDrag,
  setHeaderDragOver, endHeaderDrag (stable methods).

- App.tsx: implement those methods. endHeaderDrag(true) swaps if
  source and over are different leaves.

- LeafPane.tsx: pointerdown on .pane-toolbar (skipped if the target
  is a button/input). 5px movement threshold before drag commits to
  prevent accidental swaps when clicking a chip etc. Pointer-capture
  the toolbar so we keep getting move events even outside it. Use
  document.elementFromPoint to find the leaf under the cursor.

- CSS: source pane fades to 40% opacity during drag; target pane
  shows a 3px dashed blue outline; toolbar shows grab/grabbing
  cursors.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/layout/LeafPane.css

@@ -17,6 +17,21 @@
 .leaf.active.broadcasting {
   border-color: #ffb840;
 }
+.leaf.drag-source {
+  opacity: 0.4;
+}
+.leaf.drag-target {
+  outline: 3px dashed #5a8cd8;
+  outline-offset: -3px;
+}
+
+/* Drag handle hint on the toolbar */
+.pane-toolbar {
+  cursor: grab;
+}
+.pane-toolbar:active {
+  cursor: grabbing;
+}
 
 .pane-toolbar {
   flex: 0 0 auto;