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, 18 insertions, 0 deletions

diff --git a/frontend/src/components/LoadingSpinner.tsx b/frontend/src/components/LoadingSpinner.tsx
new file mode 100644
index 0000000..2e47f28
--- /dev/null
+++ b/frontend/src/components/LoadingSpinner.tsx
@@ -0,0 +1,18 @@
+import { faSpinner } from "@fortawesome/free-solid-svg-icons";
+import { FontAwesomeIcon } from "@fortawesome/react-fontawesome";
+
+interface LoadingSpinnerProps {
+	className?: string;
+}
+
+export function LoadingSpinner({ className = "" }: LoadingSpinnerProps) {
+	return (
+		<div className={`flex items-center justify-center py-12 ${className}`}>
+			<FontAwesomeIcon
+				icon={faSpinner}
+				className="h-8 w-8 animate-spin text-stone-400"
+				aria-hidden="true"
+			/>
+		</div>
+	);
+}