refactor(client): migrate state management from React Context to Jotai

Replace AuthProvider and SyncProvider with Jotai atoms for more granular
state management and better performance. This migration:

- Creates atoms for auth, sync, decks, cards, noteTypes, and study state
- Uses atomFamily for parameterized state (e.g., cards by deckId)
- Introduces StoreInitializer component for subscription initialization
- Updates all components and pages to use useAtomValue/useSetAtom
- Updates all tests to use Jotai Provider with createStore pattern

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/src/client/components/ErrorBoundary.tsx b/src/client/components/ErrorBoundary.tsx
new file mode 100644
index 0000000..a86ea9a
--- /dev/null
+++ b/src/client/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="bg-error/5 border border-error/20 rounded-xl p-4"
+		>
+			<span className="text-error">
+				{error?.message ?? "An error occurred"}
+			</span>
+		</div>
+	);
+}