feat(login): 增加 release 包服务端设置入口的开关控制

添加环境变量控制 release 包是否显示服务端设置入口，默认关闭，调试版仍直接显示。同时调整部分代码格式以保持一致性。

lib/features/auth/presentation/login_page.dart

@@ -14,6 +14,12 @@ import '../../../shared/widgets/x_common.dart';
 import '../../../shared/widgets/x_icon.dart';
 import '../application/session_controller.dart';
 
+/// 是否在 release 包里也显示「服务端设置」入口。
+///
+/// 默认关闭，因此正式交付包不会暴露改地址的口子；打包内部测试包时用
+/// `--dart-define=ENABLE_SERVER_SETTINGS=true` 打开，测试同事即可自行切换后端。
+const _serverSettingsEnabled = bool.fromEnvironment('ENABLE_SERVER_SETTINGS');
+
 /// 登录页，对应 `pages/login/index.vue`。
 class LoginPage extends ConsumerStatefulWidget {
   const LoginPage({super.key});
@@ -90,7 +96,10 @@ class _LoginPageState extends ConsumerState<LoginPage> {
               padding: const EdgeInsets.symmetric(horizontal: 24, vertical: 40),
               child: ConstrainedBox(
                 constraints: BoxConstraints(
-                  minHeight: (constraints.maxHeight - 80).clamp(0, double.infinity),
+                  minHeight: (constraints.maxHeight - 80).clamp(
+                    0,
+                    double.infinity,
+                  ),
                 ),
                 child: Column(
                   mainAxisAlignment: MainAxisAlignment.center,
@@ -111,8 +120,8 @@ class _LoginPageState extends ConsumerState<LoginPage> {
               ),
             ),
           ),
-          // 测试用「服务端设置」入口，仅调试版可见
-          if (kDebugMode)
+          // 测试用「服务端设置」入口：调试版常显，release 需显式开关
+          if (kDebugMode || _serverSettingsEnabled)
             Positioned(
               top: 0,
               right: 0,
@@ -142,9 +151,7 @@ class _LoginPageState extends ConsumerState<LoginPage> {
       isScrollControlled: true,
       backgroundColor: AppColors.surface,
       shape: const RoundedRectangleBorder(
-        borderRadius: BorderRadius.vertical(
-          top: Radius.circular(AppRadius.lg),
-        ),
+        borderRadius: BorderRadius.vertical(top: Radius.circular(AppRadius.lg)),
       ),
       builder: (_) => const _ServerSettingsSheet(),
     );
@@ -160,7 +167,10 @@ class _LoginPageState extends ConsumerState<LoginPage> {
             gradient: const LinearGradient(
               begin: Alignment.topLeft,
               end: Alignment.bottomRight,
-              colors: [AppColors.brandGradientStart, AppColors.brandGradientEnd],
+              colors: [
+                AppColors.brandGradientStart,
+                AppColors.brandGradientEnd,
+              ],
             ),
             borderRadius: BorderRadius.circular(20),
             boxShadow: const [
@@ -307,9 +317,7 @@ class _SoftBlob extends StatelessWidget {
         height: size,
         decoration: BoxDecoration(
           shape: BoxShape.circle,
-          gradient: RadialGradient(
-            colors: [color, color.withValues(alpha: 0)],
-          ),
+          gradient: RadialGradient(colors: [color, color.withValues(alpha: 0)]),
         ),
       ),
     );
@@ -405,7 +413,7 @@ class _ServerSettingsSheetState extends ConsumerState<_ServerSettingsSheet> {
             Text('服务端设置', style: AppTypography.titleSm),
             const SizedBox(height: 6),
             const Text(
-              '仅调试版可见。保存后立即生效，无需重启 App。',
+              '保存后立即生效，无需重启 App。',
               style: TextStyle(fontSize: 13, color: AppColors.textMuted),
             ),
             const SizedBox(height: 20),