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/Usage.test.tsx

@@ -98,7 +98,7 @@ describe('Usage page', () => {
     renderWithProviders(<Usage />)
 
     expect(screen.getByText('用量统计')).toBeInTheDocument()
-    expect(screen.getByText('查看模型使用情况和 Token 消耗')).toBeInTheDocument()
+    expect(screen.getByText('查看模型使用情况、Token 消耗和用户转化')).toBeInTheDocument()
 
     // Summary card titles
     expect(screen.getByText('总请求数')).toBeInTheDocument()