feat: pro: pane-aware responsiveness, scoped sidebar overlay, stable pane keys

hooks/use-pane-size.ts

@@ -0,0 +1,14 @@
+"use client";
+
+import { createContext, useContext } from "react";
+
+/**
+ * Width of the pane that's hosting the current subtree, in CSS pixels.
+ * `null` means "no pane is providing a size" — fall back to viewport-based
+ * media queries. Set by the Pro shell on each split pane via ResizeObserver.
+ */
+export const PaneSizeContext = createContext<number | null>(null);
+
+export function usePaneSize(): number | null {
+  return useContext(PaneSizeContext);
+}