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/lib/api.ts

@@ -10,8 +10,13 @@ class ApiError extends Error {
 
 interface RequestConfig {
   params?: Record<string, string | number | boolean | undefined>;
+  /** Caller cancellation (TanStack Query passes this); combined with the
+   * default 30s timeout. */
+  signal?: AbortSignal;
 }
 
+const REQUEST_TIMEOUT_MS = 30_000;
+
 async function request<T>(
   method: string,
   url: string,
@@ -38,11 +43,17 @@ async function request<T>(
     fetchBody = JSON.stringify(body);
   }
 
+  const timeout = AbortSignal.timeout(REQUEST_TIMEOUT_MS);
+  const signal = config?.signal
+    ? AbortSignal.any([config.signal, timeout])
+    : timeout;
+
   const res = await fetch(fullUrl, {
     method,
     headers,
     body: fetchBody,
     credentials: "same-origin",
+    signal,
   });
 
   if (res.status === 401) {