Adopt TanStack Query: provider, timeouts, useBudget, Budget page

QueryClientProvider (30s staleTime, 1 retry) + Sonner Toaster at the
root. api.ts gains AbortSignal support and a default 30s timeout
(FE-24); query cancellation kills the stale-response races (FE-01,
FE-03). useBudget is now queries + mutations with targeted ['budget']
invalidation and success/error toasts on every mutation (ARCH-13,
UX-01); month navigation keeps the previous month visible while
fetching. Budget's transaction list is a cancellable query with
placeholder data; deferred-toggle gets feedback (UX-05); both panels
render a shared ErrorState with retry instead of failing silently.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1,8 +1,19 @@
 import { BrowserRouter, Routes, Route, Navigate } from "react-router-dom";
 import { CopilotKit } from "@copilotkit/react-core";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import { Toaster } from "sonner";
 import { AuthProvider, useAuth } from "./AuthContext";
 import { ThemeProvider } from "./contexts/theme-context";
 import { PrivacyProvider } from "./contexts/privacy-context";
+
+const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: {
+      staleTime: 30_000,
+      retry: 1,
+    },
+  },
+});
 import Layout from "./components/Layout";
 import LoginPage from "./pages/Login";
 import Asistente from "./pages/Asistente";
@@ -58,9 +69,12 @@ export default function App() {
       <ThemeProvider>
         <PrivacyProvider>
           <AuthProvider>
-            <CopilotKit runtimeUrl="/api/copilotkit" agent="wealthysmart" a2ui={{}}>
-              <AppRoutes />
-            </CopilotKit>
+            <QueryClientProvider client={queryClient}>
+              <CopilotKit runtimeUrl="/api/copilotkit" agent="wealthysmart" a2ui={{}}>
+                <AppRoutes />
+                <Toaster richColors position="top-right" closeButton />
+              </CopilotKit>
+            </QueryClientProvider>
           </AuthProvider>
         </PrivacyProvider>
       </ThemeProvider>