Refactor useAuth imports to utilize AuthContext and enhance test store injection handling

- Replaced `useAuthStore` imports with `useAuth` from `AuthContext` in `AuthGuard` and `Header` for consistency.
- Enhanced `getAuthStore` to prioritize E2E test store injection for improved testability.
- Updated comments to reflect changes and clarify usage patterns.

frontend/src/components/auth/AuthGuard.tsx

@@ -8,7 +8,7 @@
 
 import { useEffect, useState } from 'react';
 import { useRouter, usePathname } from 'next/navigation';
-import { useAuth } from '@/lib/stores';
+import { useAuth } from '@/lib/auth/AuthContext';
 import { useMe } from '@/lib/api/hooks/useAuth';
 import { AuthLoadingSkeleton } from '@/components/layout';
 import config from '@/config/app.config';

frontend/src/components/layout/Header.tsx

@@ -8,7 +8,7 @@
 
 import Link from 'next/link';
 import { usePathname } from 'next/navigation';
-import { useAuth } from '@/lib/stores';
+import { useAuth } from '@/lib/auth/AuthContext';
 import { useLogout } from '@/lib/api/hooks/useAuth';
 import {
   DropdownMenu,

frontend/src/lib/api/client.ts

@@ -28,11 +28,20 @@ let refreshPromise: Promise<string> | null = null;
 /**
  * Auth store accessor
  * Dynamically imported to avoid circular dependencies
+ * Checks for E2E test store injection before using production store
  *
  * Note: Tested via E2E tests when interceptors are invoked
  */
 /* istanbul ignore next */
 const getAuthStore = async () => {
+  // Check for E2E test store injection (same pattern as AuthProvider)
+  if (typeof window !== 'undefined' && (window as any).__TEST_AUTH_STORE__) {
+    const testStore = (window as any).__TEST_AUTH_STORE__;
+    // Test store must have getState() method for non-React contexts
+    return testStore.getState();
+  }
+
+  // Production: use real Zustand store
   const { useAuthStore } = await import('@/lib/stores/authStore');
   return useAuthStore.getState();
 };