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, 18 insertions, 0 deletions

diff --git a/src/client/components/LoadingSpinner.tsx b/src/client/components/LoadingSpinner.tsx
new file mode 100644
index 0000000..95159ff
--- /dev/null
+++ b/src/client/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 text-primary animate-spin"
+				aria-hidden="true"
+			/>
+		</div>
+	);
+}