[f-forge] 页面工程师：已完成实现并验证。

```yaml
checklist:
  target_files:
    - lib/pages/login_page.dart
    - lib/widgets/primary_button.dart
  changes:
    - "ElevatedButton 颜色 blue 改为 theme.primary"
    - "提取 PrimaryButton 公共组件"
  freeze_alignment: true
  deviations: []
  regression_scope:
    - "登录按钮点击"
    - "密码找回按钮点击"
  verification_type: minimal
  commands_run:
    - "flutter analyze"
```
