Add "⇲ Dock all" button for bulk-docking popped-out panels #3

Background

This repo (per PR #20) has per-panel names, top-right Pop/Dock placement, and exposes panels to other plugins — capabilities slopsmith-plugin-splitscreen lacks. However, slopsmith has a "⇲ Dock all" button that this repo doesn't: a single control that docks every currently popped-out panel back into the main window at once, instead of requiring the user to dock pop-outs one at a time.

Ask

Port a "Dock all" button (visible only when ≥1 panel is popped out) that iterates the popups tracking structure and re-docks each one, reusing this repo's existing single-panel _redockPanel-equivalent logic per popup. Should respect the same in-flight/single-flight guards used elsewhere in the dock/pop-out lifecycle (see also the related issue about popOutPanel()'s missing _starting guard).

Reference

slopsmith-plugin-splitscreen screen.js — dockAllBtn / "Dock all pop-outs" section.