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, 15 insertions, 0 deletions

diff --git a/frontend/src/atoms/index.ts b/frontend/src/atoms/index.ts
new file mode 100644
index 0000000..fdcf7e9
--- /dev/null
+++ b/frontend/src/atoms/index.ts
@@ -0,0 +1,15 @@
+export {
+	articleFeedFilterAtom,
+	articlesInfiniteAtom,
+	articleViewAtom,
+} from "./articles";
+export {
+	authLoadingAtom,
+	isLoggedInAtom,
+	loginAtom,
+	logoutAtom,
+	useAuthInit,
+	userAtom,
+} from "./auth";
+
+export { feedsAtom } from "./feeds";