fix: 修复 Apple 登录 sub claim 映射 + 按钮文案中文化

- AppleTokenValidator: 从 JwtSecurityToken 原始 Claims 取 sub，避免 .NET ClaimType 映射导致丢失
- 登录页 Apple 按钮改为中文"通过 Apple 登录"

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

health_app/lib/pages/auth/login_page.dart

@@ -563,6 +563,7 @@ class _LoginPageState extends ConsumerState<LoginPage> {
                                       onPressed: _loading ? () {} : () { _appleSignIn(); },
                                       style: SignInWithAppleButtonStyle.whiteOutlined,
                                       height: 48,
+                                      text: '通过 Apple 登录',
                                     ),
                                   ],
                                   const SizedBox(height: 16),