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, 42 insertions, 0 deletions

diff --git a/frontend/src/components/ErrorBoundary.tsx b/frontend/src/components/ErrorBoundary.tsx
new file mode 100644
index 0000000..7f06085
--- /dev/null
+++ b/frontend/src/components/ErrorBoundary.tsx
@@ -0,0 +1,42 @@
+import { Component, type ReactNode } from "react";
+
+interface ErrorBoundaryProps {
+	children: ReactNode;
+	fallback?: ReactNode;
+}
+
+interface ErrorBoundaryState {
+	hasError: boolean;
+	error: Error | null;
+}
+
+export class ErrorBoundary extends Component<
+	ErrorBoundaryProps,
+	ErrorBoundaryState
+> {
+	override state: ErrorBoundaryState = { hasError: false, error: null };
+
+	static getDerivedStateFromError(error: Error): ErrorBoundaryState {
+		return { hasError: true, error };
+	}
+
+	override render() {
+		if (this.state.hasError) {
+			return this.props.fallback ?? <ErrorFallback error={this.state.error} />;
+		}
+		return this.props.children;
+	}
+}
+
+function ErrorFallback({ error }: { error: Error | null }) {
+	return (
+		<div
+			role="alert"
+			className="rounded-lg border border-red-200 bg-red-50 p-4"
+		>
+			<span className="text-sm text-red-600">
+				{error?.message ?? "An error occurred"}
+			</span>
+		</div>
+	);
+}