Fix detached-window IPC scoping and pane-transfer session loss

- capabilities/default.json: extend window scope to "pane-window-*" so
  detached windows can invoke/listen (fixes blank panes B2-B5).
- App.tsx: memoize the destructive take_pending_window_init read at module
  scope so React StrictMode's double mount-effect doesn't consume the
  transfer payload twice and lose the adopted PTY session.
- lib.rs: add `use tauri::Manager;` for Window::app_handle() in on_window_event.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src-tauri/capabilities/default.json

@@ -2,7 +2,7 @@
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
   "description": "Default capability set for wsl-mux spike",
-  "windows": ["main"],
+  "windows": ["main", "pane-window-*"],
   "permissions": [
     "core:default",
     "core:event:default",