feat: add query provider to App.svelte

src/app/App.svelte

@@ -6,13 +6,17 @@
  * layout shell. This is the root component mounted by the application.
  *
  * Structure:
+ * - QueryProvider provides TanStack Query client for data fetching
  * - Layout provides sidebar, header/footer, and page container
  * - Page renders the current route content
  */
 import Page from '$routes/Page.svelte';
+import { QueryProvider } from './providers';
 import Layout from './ui/Layout.svelte';
 </script>
 
-<Layout>
-    <Page />
-</Layout>
+<QueryProvider>
+    <Layout>
+        <Page />
+    </Layout>
+</QueryProvider>