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/frontend/src/components/app-shell.tsx

@@ -219,7 +219,7 @@ const PAGE_TO_NAV: Partial<Record<Page, Page>> = {
   settingsNotify: "settings"
 };
 
-export function Sidebar({ page, navigate, user, team, products, projects, productTotal, projectTotal, logout }: {
+export function Sidebar({ page, navigate, user, team, products, projects, productTotal, projectTotal, logout, onOpenAdmin }: {
   page: Page;
   navigate: Navigate;
   user: User;
@@ -231,6 +231,8 @@ export function Sidebar({ page, navigate, user, team, products, projects, produc
   // 退出登录。父级(App.tsx)有完整 logout 闭包时传入;未传时用自带兜底
   // (best-effort 调登出接口 + 清 token + 跳 /login),保证侧栏账户菜单的"退出"今天就可用。
   logout?: () => void;
+  // 平台超管入口:仅 user.is_platform_admin 时显示,点了进 /admin 后台
+  onOpenAdmin?: () => void;
 }) {
   const activeNav = PAGE_TO_NAV[page];
   // 徽标用后端真实总数(分页 count),不用已加载的页内条数
@@ -330,6 +332,22 @@ export function Sidebar({ page, navigate, user, team, products, projects, produc
           </a>
         ))}
       </nav>
+      {user.is_platform_admin && onOpenAdmin && (
+        <>
+          <div className="nav-section">平台</div>
+          <nav>
+            <a
+              href="/admin"
+              title="平台后台"
+              aria-label="平台后台"
+              onClick={(event) => { event.preventDefault(); setMobileNavOpen(false); onOpenAdmin(); }}
+            >
+              <IconKitSvg name="shield" />
+              <span>平台后台</span>
+            </a>
+          </nav>
+        </>
+      )}
       <div className="aside-foot">
         <div
           className="user"