refactor(frontend): migrate state management to jotai and jotai-tanstack-query

Replace React Context + manual useEffect data fetching with jotai atoms
for state management and jotai-tanstack-query for server state caching.

- Add jotai, jotai-tanstack-query, @tanstack/query-core dependencies
- Create atoms for auth (primitive + action), feeds (suspense query),
  and articles (infinite query with cursor-based pagination)
- Wire up Provider, HydrateQueryClient, and StoreInitializer in main.tsx
- Migrate all components from useAuth() context to jotai atoms
- Replace manual fetch logic in FeedSidebar/FeedList with feedsAtom
- Replace usePaginatedArticles hook with articlesInfiniteAtom
- Add queryClient.invalidateQueries() after mutations for automatic
  cache refresh
- Add ErrorBoundary and LoadingSpinner components for Suspense support
- Remove callback prop chains (onFeedAdded, onFeedChanged, etc.)
  in favor of query invalidation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/frontend/src/components/index.ts b/frontend/src/components/index.ts
index c0797b4..e10b0b8 100644
--- a/frontend/src/components/index.ts
+++ b/frontend/src/components/index.ts
@@ -1,8 +1,11 @@
 export { AddFeedForm } from "./AddFeedForm";
 export { ArticleList } from "./ArticleList";
+export { ErrorBoundary } from "./ErrorBoundary";
 export { FeedList } from "./FeedList";
 export { FeedSidebar } from "./FeedSidebar";
 export { Layout } from "./Layout";
+export { LoadingSpinner } from "./LoadingSpinner";
 export { MenuItem } from "./MenuItem";
 export { Navigation } from "./Navigation";
 export { ProtectedRoute } from "./ProtectedRoute";
+export { StoreInitializer } from "./StoreInitializer";