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/components/ErrorState.tsx

@@ -0,0 +1,28 @@
+import { AlertTriangle, RotateCw } from 'lucide-react';
+import { Button } from '@/components/ui/button';
+
+interface Props {
+  message?: string;
+  onRetry?: () => void;
+}
+
+export default function ErrorState({
+  message = 'No se pudieron cargar los datos',
+  onRetry,
+}: Props) {
+  return (
+    <div
+      role="alert"
+      className="flex flex-col items-center justify-center gap-3 rounded-lg border border-destructive/40 bg-destructive/5 p-8 text-center"
+    >
+      <AlertTriangle className="h-6 w-6 text-destructive" aria-hidden="true" />
+      <p className="text-sm text-muted-foreground">{message}</p>
+      {onRetry && (
+        <Button variant="outline" size="sm" onClick={onRetry}>
+          <RotateCw className="mr-2 h-4 w-4" aria-hidden="true" />
+          Reintentar
+        </Button>
+      )}
+    </div>
+  );
+}