继续收口认证入口弹窗壳层

新增 PlatformAuthModalShell 统一认证白底弹窗壳层

登录入口和邀请码弹窗复用共享认证壳层

补充认证壳层和 AuthGate 接入测试

同步 PlatformUiKit 文档和 Hermes 决策记录

src/components/auth/PlatformAuthModalShell.tsx

@@ -0,0 +1,59 @@
+import type { ReactNode } from 'react';
+
+import type { PlatformTheme } from '../../../packages/shared/src/contracts/runtime';
+import { UnifiedModal } from '../common/UnifiedModal';
+
+type PlatformAuthModalShellProps = {
+  title: string;
+  platformTheme: PlatformTheme;
+  onClose: () => void;
+  children: ReactNode;
+  closeLabel: string;
+  zIndexClassName?: string;
+  panelClassName?: string;
+  bodyClassName?: string;
+};
+
+function joinClassNames(...classNames: Array<string | false | null | undefined>) {
+  return classNames.filter(Boolean).join(' ');
+}
+
+/**
+ * 认证入口弹窗共享壳层。
+ * 这里只统一主题遮罩、auth card、标题栏和关闭按钮，登录 / 邀请码表单状态仍留在各自业务组件。
+ */
+export function PlatformAuthModalShell({
+  title,
+  platformTheme,
+  onClose,
+  children,
+  closeLabel,
+  zIndexClassName = 'z-[120]',
+  panelClassName,
+  bodyClassName = '!p-0',
+}: PlatformAuthModalShellProps) {
+  return (
+    <UnifiedModal
+      open
+      title={title}
+      onClose={onClose}
+      closeLabel={closeLabel}
+      closeVariant="platformIcon"
+      closeOnBackdrop
+      closeOnEscape={false}
+      portal={false}
+      size="sm"
+      zIndexClassName={zIndexClassName}
+      overlayClassName={`platform-theme platform-theme--${platformTheme} !px-3 !py-4 text-[var(--platform-text-strong)] sm:!p-4`}
+      panelClassName={joinClassNames(
+        'platform-auth-card !rounded-[2rem] sm:!rounded-[2rem]',
+        panelClassName,
+      )}
+      headerClassName="!items-center !px-5 !py-4"
+      titleClassName="text-lg font-semibold text-[var(--platform-text-strong)]"
+      bodyClassName={bodyClassName}
+    >
+      {children}
+    </UnifiedModal>
+  );
+}