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/tree.ts

@@ -356,6 +356,28 @@ export function updateSplitRatio(root: TreeNode, splitId: NodeId, ratio: number)
   });
 }
 
+/** Swap two leaves' tree positions. Each leaf carries its own data
+ *  (id, distro, cwd, label, broadcast) into the other's slot. PTYs stay
+ *  alive because React keys on leaf.id and our renderer is flat. */
+export function swapLeaves(root: TreeNode, idA: NodeId, idB: NodeId): TreeNode {
+  if (idA === idB) return root;
+  const a = findLeaf(root, idA);
+  const b = findLeaf(root, idB);
+  if (!a || !b) return root;
+  function walk(n: TreeNode): TreeNode {
+    if (n.kind === "leaf") {
+      if (n.id === idA) return b!;
+      if (n.id === idB) return a!;
+      return n;
+    }
+    const na = walk(n.a);
+    const nb = walk(n.b);
+    if (na === n.a && nb === n.b) return n;
+    return { ...n, a: na, b: nb };
+  }
+  return walk(root);
+}
+
 export function serialize(root: TreeNode): string {
   return JSON.stringify(root);
 }