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, 57 insertions, 0 deletions

diff --git a/src/client/atoms/auth.ts b/src/client/atoms/auth.ts
new file mode 100644
index 0000000..f618ccf
--- /dev/null
+++ b/src/client/atoms/auth.ts
@@ -0,0 +1,57 @@
+import { atom, useSetAtom } from "jotai";
+import { useEffect } from "react";
+import { apiClient, type User } from "../api/client";
+
+// Primitive atoms
+export const userAtom = atom<User | null>(null);
+export const authLoadingAtom = atom<boolean>(true);
+
+// Derived atom - checks if user is authenticated via apiClient
+export const isAuthenticatedAtom = atom<boolean>((get) => {
+	// We need to trigger re-evaluation when user changes
+	get(userAtom);
+	return apiClient.isAuthenticated();
+});
+
+// Action atom - login
+export const loginAtom = atom(
+	null,
+	async (
+		_get,
+		set,
+		{ username, password }: { username: string; password: string },
+	) => {
+		const response = await apiClient.login(username, password);
+		set(userAtom, response.user);
+	},
+);
+
+// Action atom - logout
+export const logoutAtom = atom(null, (_get, set) => {
+	apiClient.logout();
+	set(userAtom, null);
+});
+
+// Hook to initialize auth state and subscribe to session expiration
+export function useAuthInit() {
+	const setAuthLoading = useSetAtom(authLoadingAtom);
+	const setUser = useSetAtom(userAtom);
+
+	useEffect(() => {
+		// Check for existing auth on mount
+		const tokens = apiClient.getTokens();
+		if (tokens) {
+			// We have tokens stored, but we don't have user info cached
+			// For now, just set authenticated state. User info will be fetched when needed.
+		}
+		setAuthLoading(false);
+
+		// Subscribe to session expired events from the API client
+		const unsubscribe = apiClient.onSessionExpired(() => {
+			apiClient.logout();
+			setUser(null);
+		});
+
+		return unsubscribe;
+	}, [setAuthLoading, setUser]);
+}