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/package.json

@@ -19,6 +19,7 @@
     "@fontsource-variable/ibm-plex-sans": "^5.2.8",
     "@fontsource-variable/noto-sans": "^5.2.10",
     "@hono/node-server": "^1.14.4",
+    "@tanstack/react-query": "^5.101.0",
     "@tanstack/react-table": "^8.21.3",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
@@ -30,6 +31,7 @@
     "react-router-dom": "^7.6.0",
     "recharts": "^3.8.1",
     "rxjs": "^7.8.1",
+    "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0",
     "tsx": "^4.19.4",
     "tw-animate-css": "^1.4.0"