Phase 2: drag-/right-click-a-pane-to-new-window

Right-click any pane's title bar → "Move to new window" pops it into a
fresh tiletopia window with its PTY intact. Same Tauri process; the
PtyManager is shared, so the existing PaneId stays valid and Tauri 2's
process-wide event routing keeps pane://{id}/data flowing into the new
window's XtermPane.

Mechanism (Rust-side, plan-agent's main correction over my draft):
- pty.rs: PtyManager.transferring is a per-pane refcount; kill_pane
  becomes a no-op while it's >0. Source window's React unmount calls
  kill_pane → silently dropped while in flight; target window's
  claim_pane decrements after it has subscribed.
- window_state.rs: per-window workspaces snapshot map +
  debounced-by-tokio aggregate save. Each window pushes its tabs via
  push_window_workspaces; backend writes the merged
  { version: 2, workspaces: [...] } envelope. Non-main windows have
  their entries dropped on CloseRequested so closing a detached window
  discards its tabs (Chrome-style).
- commands: mark_pane_transferring, claim_pane, get_pane_ring (base64
  scrollback ring snapshot), create_pane_window, take_pending_window_init,
  push_window_workspaces.

Frontend:
- XtermPane gets `existingPaneId?: PaneId`: skip spawn, replay ring
  snapshot via term.write before attaching the live data listener,
  resize PTY to this window's grid, claim_pane. Scrollback replay was
  the plan agent's other ship-in-v1 call — without it a transferred
  Claude session looks blank until next prompt repaint.
- LeafPane: onContextMenu opens a fixed-positioned "Move to new
  window" popover. Esc / outside-click dismiss.
- orchestration adds moveToNewWindow + getInitialPaneIdFor; App owns a
  one-shot transferredPaneIdsRef cleared in registerPaneId.
- App mount branches on getCurrentWebviewWindow().label: main loads
  workspace.json as before; non-main calls take_pending_window_init
  and builds a singleton workspace around the adopted leaf.
- MCP mirror + onMcpRequest only run in main (paneIdByLeafRef is per-
  window; Claude sees the main window's current tab as the single
  workspace surface).

pnpm check (tsc -b) clean. 79/79 vitest pass. Rust side authored in
WSL; cargo build needs verification on Windows host before this is
runnable.

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

src/lib/layout/LeafPane.tsx

@@ -185,6 +185,38 @@ export default function LeafPane({ leaf }: { leaf: LeafNode }) {
     setStatusOk(ok);
   }, []);
 
+  // ---- right-click context menu ------------------------------------------
+  // Single entry in v1: "Move to new window" (pops the pane out into a
+  // fresh top-level tiletopia window without losing the PTY).
+  const [menuPos, setMenuPos] = useState<{ x: number; y: number } | null>(null);
+  const openContextMenu = useCallback(
+    (e: MouseEvent<HTMLDivElement>) => {
+      e.preventDefault();
+      e.stopPropagation();
+      setMenuPos({ x: e.clientX, y: e.clientY });
+    },
+    [],
+  );
+  const closeContextMenu = useCallback(() => setMenuPos(null), []);
+  useEffect(() => {
+    if (!menuPos) return;
+    const onDocClick = () => setMenuPos(null);
+    const onEsc = (e: globalThis.KeyboardEvent) => {
+      if (e.key === "Escape") setMenuPos(null);
+    };
+    // Defer attaching the click listener so the click that opened the menu
+    // doesn't immediately close it.
+    const t = window.setTimeout(() => {
+      window.addEventListener("click", onDocClick);
+      window.addEventListener("keydown", onEsc, true);
+    }, 0);
+    return () => {
+      clearTimeout(t);
+      window.removeEventListener("click", onDocClick);
+      window.removeEventListener("keydown", onEsc, true);
+    };
+  }, [menuPos]);
+
   // ---- header-drag swap ---------------------------------------------------
   // Drag the toolbar onto another pane's toolbar/body to swap their tree
   // positions. Uses a movement threshold so accidental tiny moves while
@@ -306,6 +338,7 @@ export default function LeafPane({ leaf }: { leaf: LeafNode }) {
         onPointerMove={onToolbarPointerMove}
         onPointerUp={onToolbarPointerUp}
         onPointerCancel={onToolbarPointerCancel}
+        onContextMenu={openContextMenu}
       >
         {editingLabel ? (
           <input
@@ -482,6 +515,7 @@ export default function LeafPane({ leaf }: { leaf: LeafNode }) {
         {spec ? (
           <XtermPane
             spec={spec}
+            existingPaneId={orch.getInitialPaneIdFor(leaf.id)}
             onStatus={onStatus}
             onSpawn={onPaneSpawned}
             onInput={onTerminalInput}
@@ -500,6 +534,31 @@ export default function LeafPane({ leaf }: { leaf: LeafNode }) {
           </div>
         )}
       </div>
+      {menuPos && (
+        <div
+          className="pane-context-menu"
+          style={{
+            position: "fixed",
+            top: menuPos.y,
+            left: menuPos.x,
+          }}
+          role="menu"
+          onClick={(e) => e.stopPropagation()}
+          onContextMenu={(e) => e.preventDefault()}
+        >
+          <button
+            type="button"
+            className="pane-context-menu-item"
+            role="menuitem"
+            onClick={() => {
+              closeContextMenu();
+              orch.moveToNewWindow(leaf.id);
+            }}
+          >
+            Move to new window
+          </button>
+        </div>
+      )}
     </div>
   );
 }