Standardize store exports to named useXxxStore convention

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/stores/login.test.js

@@ -13,7 +13,7 @@ vi.mock('@/api/client.js', () => ({
   default: { get: mockClientGet },
 }));
 
-import useLoginStore from '@/stores/login.ts';
+import { useLoginStore } from '@/stores/login.ts';
 
 // Mock axios methods (used for signIn/refreshToken which call plain axios)
 vi.spyOn(axios, 'post').mockImplementation(vi.fn());