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/Prompts.test.tsx

@@ -92,8 +92,7 @@ describe('Prompts page', () => {
 
     renderWithProviders(<Prompts />)
 
-    expect(screen.getByText('提示词管理')).toBeInTheDocument()
-    expect(screen.getByText('管理系统提示词模板和版本历史')).toBeInTheDocument()
+    // "新建提示词" button is rendered by toolBarRender
     expect(screen.getByText('新建提示词')).toBeInTheDocument()
   })