feat(admin): Phase 0 平台超管基础 — is_platform_admin + IsPlatformAdmin + create_platform_admin(admin/admin123) + AdminAuditLog + Admin 后台外壳与路由 gating

后端:User.is_platform_admin + migration;权限类 IsPlatformAdmin;管理命令建 admin/admin123(幂等);
AdminAuditLog 模型 + log_admin_action() helper;me/login 对无团队超管优雅返回 team=null;UserSerializer 暴露标志。
前端:routes/admin 后台外壳(分组侧栏 + 概览 + 占位)、/admin 路由解析与 gating(超管直落、非超管纠回)、
侧栏平台入口、admin-page.css(仅 token)、IconKitSvg 补图标。
测试:accounts 11/11 单测过;无头 e2e _admin-p0.mjs 全断言过 + 0 console error;tsc+build 绿。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

core/backend/apps/accounts/serializers.py

@@ -8,8 +8,11 @@ from .models import Invitation, LoginSession, Team, TeamMember, User, UserPrefer
 class UserSerializer(serializers.ModelSerializer):
     class Meta:
         model = User
-        fields = ["id", "username", "first_name", "last_name", "email", "phone", "avatar_url", "status"]
-        read_only_fields = ["id", "status"]
+        fields = [
+            "id", "username", "first_name", "last_name", "email", "phone",
+            "avatar_url", "status", "is_platform_admin",
+        ]
+        read_only_fields = ["id", "status", "is_platform_admin"]
 
 
 class UserPreferenceSerializer(serializers.ModelSerializer):