test(admin): sync page tests with component changes (BUG-007)

Fix 6 page test files to match actual component output:
- Login: cookie-based auth login(account), brand text updates
- Config/Logs/Prompts: remove stale description text assertions
- ModelServices: check for actual table buttons instead of title
- Usage: update description text to match PageHeader

All 132 tests pass (17/17 files).

admin-v2/tests/pages/Config.test.tsx

@@ -101,7 +101,6 @@ describe('Config page', () => {
     renderWithProviders(<Config />)
 
     expect(screen.getByText('系统配置')).toBeInTheDocument()
-    expect(screen.getByText('管理系统运行参数和功能开关')).toBeInTheDocument()
   })
 
   it('fetches and displays config items', async () => {