feat: Integrate Expo mobile application into monorepo (#16)

- Add mobile/ directory with Expo React Native project
- Create API client with JWT auth and token refresh using AsyncStorage
- Implement AuthContext for login/register/logout flow
- Add screens: Login, Register, Library, Search, Settings
- Set up React Navigation with AuthStack and MainTabs
- Create packages/shared/ with shared types and utilities
- Add shared validation utilities (email, password strength)
- Update root package.json workspaces to include mobile + shared
- Add spec document docs/backend/009-expo-integration.md

mobile/src/hooks/useAsyncData.ts

@@ -0,0 +1,33 @@
+import { useState, useEffect, useCallback } from "react";
+
+/**
+ * Generic async data fetching hook for mobile screens.
+ */
+export function useAsyncData<T>(
+  fetcher: () => Promise<T>,
+  deps: unknown[] = [],
+) {
+  const [data, setData] = useState<T | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState<Error | null>(null);
+
+  const execute = useCallback(async () => {
+    setLoading(true);
+    setError(null);
+    try {
+      const result = await fetcher();
+      setData(result);
+    } catch (err) {
+      setError(err instanceof Error ? err : new Error(String(err)));
+    } finally {
+      setLoading(false);
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, deps);
+
+  useEffect(() => {
+    execute();
+  }, [execute]);
+
+  return { data, loading, error, refetch: execute };
+}