feat: add shared toolbox header and appearance

packages/shell-react/README.md

@@ -4,7 +4,7 @@
 import { AppShell } from "@add-ideas/toolbox-shell-react";
 import "@add-ideas/toolbox-shell-react/styles.css";
 
-<AppShell app={manifest} appActions={<a href="/help">Help</a>}>
+<AppShell app={manifest} helpAction={{ onClick: () => setHelpOpen(true) }}>
   <App />
 </AppShell>;
 ```
@@ -13,3 +13,6 @@ import "@add-ideas/toolbox-shell-react/styles.css";
 standalone mode if context is absent, unavailable, cross-origin, or invalid. Its
 app switcher is exposed as the accessible navigation landmark
 `Toolbox applications` and all destinations use ordinary document navigation.
+The shared top bar also exposes Help, Gitea source, light/dark/system
+personalization, and Apps controls. `ToolboxHeader` is exported separately for
+custom shells.