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/App.tsx

@@ -1,6 +1,6 @@
 import { Routes, Route} from 'react-router';
 import Home from './pages/home.tsx';
-import Inventory from "./pages/inventory.tsx";
+import InventoryList from "./features/inventory/InventoryList.tsx";
 import { QueryClientProvider, QueryClient } from '@tanstack/react-query';
 
 // Create a client
@@ -12,7 +12,7 @@ function App() {
     <>
         <Routes>
             <Route index element={<Home />} />
-            <Route path="inventory" element={<Inventory />} />
+            <Route path="inventory" element={<InventoryList />} />
         </Routes>