feat(web): 店铺管理页与页头平台选择器接入真实 API

- 页头平台选择器：PlatformProvider 加载平台目录与平台上下文
  （X-Atlas-Platform-Id header），平台 id 仅作为非敏感 UI 偏好存
  localStorage，默认取目录第一家（Ozon 优先）
- 店铺管理页覆盖 docs/api/stores-and-platforms.md 已实现接口：
  游标分页列表、创建（202+operation）、编辑（If-Match 乐观锁、
  409 刷新提示）、设为默认、重新验证凭据、仓库同步（422 脱敏
  错误展示）、软删除（DEFAULT_STORE_REPLACEMENT_REQUIRED 提示）
- services/api.ts 新增 getList（保留 {data, meta} 分页信封）与
  apiErrorCode；services/stores.ts 按契约生成 Idempotency-Key
- @atlas/contracts 补充只读响应类型 Platform / PlatformContext /
  CursorPaginationMeta / StoreOperationResponse（无行为变更）
- API Key 仅存在于提交瞬间的内存表单，不落任何存储
- 浏览器实测：注册→平台切换→创建/编辑/设默认/删除保护/同步
  错误路径→刷新会话恢复全部通过；typecheck + production build 通过

apps/web/src/services/platform.tsx

@@ -0,0 +1,145 @@
+import {
+    createContext,
+    useCallback,
+    useContext,
+    useEffect,
+    useMemo,
+    useRef,
+    useState,
+} from 'react';
+import type { ReactNode } from 'react';
+import type { Platform, PlatformContext } from '@atlas/contracts';
+import { api, apiErrorMessage } from '@/services/api';
+
+/** 平台选择的本地记忆键：仅保存平台 id 这一非敏感 UI 偏好 */
+const PLATFORM_STORAGE_KEY = 'atlas.platform_id';
+
+interface PlatformContextValue {
+    /** 已启用的平台目录；加载失败时为空数组并置 error */
+    platforms: Platform[];
+    /** 页头当前选中的平台；目录未加载完成或为空时为 null */
+    currentPlatform: Platform | null;
+    /** 目录加载失败信息；非空时提供重试入口 */
+    error: string | null;
+    /** 当前平台的默认门店上下文（GET /api/platform-context） */
+    context: PlatformContext | null;
+    contextLoading: boolean;
+    selectPlatform: (platformId: string) => void;
+    /** 门店变更后刷新默认门店摘要 */
+    refreshContext: () => Promise<void>;
+    /** 目录加载失败后重试 */
+    reload: () => void;
+}
+
+const PlatformContextContext = createContext<PlatformContextValue | null>(null);
+
+/** 平台上下文：页头平台选择器的数据源，门店等模块据此确定当前平台（目录接口将 Ozon 排在最前） */
+export function PlatformProvider({ children }: { children: ReactNode }) {
+    const [platforms, setPlatforms] = useState<Platform[]>([]);
+    const [selectedId, setSelectedId] = useState<string | null>(() =>
+        localStorage.getItem(PLATFORM_STORAGE_KEY)
+    );
+    const [error, setError] = useState<string | null>(null);
+    const [context, setContext] = useState<PlatformContext | null>(null);
+    const [contextLoading, setContextLoading] = useState(false);
+    // 快速切换平台时丢弃过期响应与 loading：只接受最后一次请求的结果
+    const contextRequestIdRef = useRef(0);
+
+    const loadPlatforms = useCallback(async () => {
+        setError(null);
+        try {
+            const list = await api.get<Platform[]>('/platforms');
+            setPlatforms(list);
+            // 已保存的选择失效（平台下线或首次使用）时回退到目录第一家（Ozon 优先）
+            setSelectedId((prev) =>
+                prev && list.some((item) => item.id === prev) ? prev : (list[0]?.id ?? null)
+            );
+        } catch (err) {
+            setError(apiErrorMessage(err));
+        }
+    }, []);
+
+    useEffect(() => {
+        void loadPlatforms();
+    }, [loadPlatforms]);
+
+    const currentPlatform = useMemo(
+        () => platforms.find((item) => item.id === selectedId) ?? null,
+        [platforms, selectedId]
+    );
+
+    const loadContext = useCallback(async (platformId: string) => {
+        const requestId = ++contextRequestIdRef.current;
+        setContextLoading(true);
+        try {
+            const next = await api.get<PlatformContext>('/platform-context', {
+                headers: { 'X-Atlas-Platform-Id': platformId },
+            });
+            if (contextRequestIdRef.current !== requestId) return;
+            setContext(next);
+        } catch {
+            // 上下文读取失败不阻塞页面：门店页等业务接口自会报错
+            if (contextRequestIdRef.current !== requestId) return;
+            setContext(null);
+        } finally {
+            if (contextRequestIdRef.current === requestId) {
+                setContextLoading(false);
+            }
+        }
+    }, []);
+
+    useEffect(() => {
+        if (!currentPlatform) {
+            setContext(null);
+            return;
+        }
+        void loadContext(currentPlatform.id);
+    }, [currentPlatform, loadContext]);
+
+    const selectPlatform = useCallback((platformId: string) => {
+        setSelectedId(platformId);
+        localStorage.setItem(PLATFORM_STORAGE_KEY, platformId);
+    }, []);
+
+    const refreshContext = useCallback(async () => {
+        if (!currentPlatform) return;
+        await loadContext(currentPlatform.id);
+    }, [currentPlatform, loadContext]);
+
+    const value = useMemo(
+        () => ({
+            platforms,
+            currentPlatform,
+            error,
+            context,
+            contextLoading,
+            selectPlatform,
+            refreshContext,
+            reload: () => void loadPlatforms(),
+        }),
+        [
+            platforms,
+            currentPlatform,
+            error,
+            context,
+            contextLoading,
+            selectPlatform,
+            refreshContext,
+            loadPlatforms,
+        ]
+    );
+
+    return (
+        <PlatformContextContext.Provider value={value}>
+            {children}
+        </PlatformContextContext.Provider>
+    );
+}
+
+export function usePlatform(): PlatformContextValue {
+    const context = useContext(PlatformContextContext);
+    if (!context) {
+        throw new Error('usePlatform 必须在 PlatformProvider 内使用');
+    }
+    return context;
+}