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>

backend/src/Health.Infrastructure/Services/apple_token_validator.cs

@@ -49,8 +49,9 @@ public sealed class AppleTokenValidator
             ClockSkew = TimeSpan.FromMinutes(1),
         };
 
-        var principal = handler.ValidateToken(identityToken, validationParams, out _);
+        handler.ValidateToken(identityToken, validationParams, out _);
-        var sub = principal.FindFirst("sub")?.Value;
+        // 从原始 JWT 取 sub（避免 .NET 的 ClaimType 映射把 "sub" 转成 NameIdentifier）
+        var sub = rawToken.Claims.FirstOrDefault(c => c.Type == "sub")?.Value;
         if (string.IsNullOrWhiteSpace(sub))
             throw new SecurityTokenException("Missing 'sub' claim in Apple identityToken");
 

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),