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/views.py

@@ -29,10 +29,21 @@ def auth_payload(user, team, token):
     return {
         "token": token.key,
         "user": UserSerializer(user).data,
-        "team": TeamSerializer(team).data,
+        "team": TeamSerializer(team).data if team is not None else None,
     }
 
 
+def current_team_or_none(user):
+    """平台超管可不属于任何团队;此时取团队返回 None(而非抛 PermissionDenied)。
+    普通用户无团队仍属异常,但后台账号体系下登录/资料接口不应因此 500。"""
+    from rest_framework.exceptions import PermissionDenied
+
+    try:
+        return get_current_team(user)
+    except PermissionDenied:
+        return None
+
+
 def _client_ip(request):
     forwarded = request.META.get("HTTP_X_FORWARDED_FOR", "")
     if forwarded:
@@ -84,7 +95,7 @@ def login(request):
     )
     if user is None or user.is_disabled:
         return Response({"detail": "invalid credentials"}, status=status.HTTP_400_BAD_REQUEST)
-    team = get_current_team(user)
+    team = current_team_or_none(user)
     token, _ = Token.objects.get_or_create(user=user)
     record_login_session(request, user)
     return Response(auth_payload(user, team, token))
@@ -110,11 +121,11 @@ def me(request):
         if email:
             user.email = email
         user.save(update_fields=["first_name", "phone", "email"])
-    team = get_current_team(user)
+    team = current_team_or_none(user)
     return Response(
         {
             "user": UserSerializer(user).data,
-            "team": TeamSerializer(team).data,
+            "team": TeamSerializer(team).data if team is not None else None,
         }
     )