Refactor inventory functionality and improve modularity.

Relocated inventory page to features directory and renamed it for clarity. Introduced a reusable `Button` component and replaced inline buttons across components. Added type definitions for inventory items and environment variable support for API URL configuration.

barkmanui/src/common/components/Button.tsx

@@ -0,0 +1,9 @@
+import {PropsWithChildren} from "react";
+
+function Button({children}: PropsWithChildren) {
+   return (
+    <button className="bg-brand rounded text-white font-bold px-4 py-2">{children}</button>
+   )
+}
+
+export default Button