feat(brand): 新增品牌切换系统核心模块

- 定义 Brand 类型、BrandCopy 文案映射、BRAND_COPY 配置
- BrandProvider + useBrand hook 提供 brand/copy/rootClassName
- BrandSessionInitializer 从 URL ?isSxwz= 初始化品牌会话
- sessionStorage 持久化 + URL 参数优先级解析
- parseBrandFromSearchParams 区分为 true/false/无参数三种情况
- 新增 default 品牌 Logo (coxwork.png)

frontend/src/core/brand/brand-session.test.ts

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+import assert from "node:assert/strict";
+import test from "node:test";
+
+const {
+  BRAND_SESSION_STORAGE_KEY,
+  getBrandRootClassName,
+  parseBrandFromSearchParams,
+  resolveBrandSession,
+} = await import(new URL("./index.ts", import.meta.url).href);
+
+void test("parseBrandFromSearchParams returns correct brand per param value", () => {
+  assert.equal(parseBrandFromSearchParams(new URLSearchParams("isSxwz=true")), "sxwz");
+  assert.equal(parseBrandFromSearchParams(new URLSearchParams("isSxwz=false")), "default");
+  assert.equal(parseBrandFromSearchParams(new URLSearchParams("")), null);
+});
+
+void test("resolveBrandSession falls back to default without url or storage", () => {
+  assert.equal(resolveBrandSession({ urlBrand: null, storedBrand: null }), "default");
+});
+
+void test("resolveBrandSession keeps stored sxwz when later url omits the flag", () => {
+  assert.equal(resolveBrandSession({ urlBrand: null, storedBrand: "sxwz" }), "sxwz");
+});
+
+void test("resolveBrandSession downgrades stored sxwz when url explicitly sets isSxwz=false", () => {
+  const urlBrand = parseBrandFromSearchParams(new URLSearchParams("isSxwz=false"));
+  assert.equal(resolveBrandSession({ urlBrand, storedBrand: "sxwz" }), "default");
+});
+
+void test("resolveBrandSession upgrades to sxwz when url flag is true", () => {
+  const urlBrand = parseBrandFromSearchParams(new URLSearchParams("isSxwz=true"));
+  assert.equal(resolveBrandSession({ urlBrand, storedBrand: "default" }), "sxwz");
+});
+
+void test("getBrandRootClassName returns stable workspace hook classes", () => {
+  assert.equal(getBrandRootClassName("default"), "brand-default");
+  assert.equal(getBrandRootClassName("sxwz"), "brand-sxwz");
+  assert.equal(BRAND_SESSION_STORAGE_KEY, "deerflow.brand-session");
+});

frontend/src/core/brand/index.ts

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+export const BRAND_SESSION_STORAGE_KEY = "deerflow.brand-session";
+export const DEFAULT_BRAND = "default" as const;
+const SXWZ_BRAND = "sxwz" as const;
+
+export type Brand = typeof DEFAULT_BRAND | typeof SXWZ_BRAND;
+
+export type BrandCopy = {
+  productLabel: string;
+  appName: string;
+  appLogoSrc?: string;
+  appLogoAlt?: string;
+};
+
+export const BRAND_COPY: Record<Brand, BrandCopy> = {
+  default: {
+    productLabel: "轻办公",
+    appName: "coxworker",
+    appLogoSrc: "/coxwork.png",
+    appLogoAlt: "coxworker",
+  },
+  sxwz: {
+    productLabel: "在线教育智能体",
+    appName: "coxstudy",
+  },
+};
+
+export function isBrand(value: string | null): value is Brand {
+  return value === DEFAULT_BRAND || value === SXWZ_BRAND;
+}
+
+export function parseBrandFromSearchParams(
+  searchParams: URLSearchParams,
+): Brand | null {
+  const value = searchParams.get("isSxwz");
+  if (value === "true") return SXWZ_BRAND;
+  if (value === "false") return DEFAULT_BRAND;
+  return null;
+}
+
+export function resolveBrandSession({
+  urlBrand,
+  storedBrand,
+}: {
+  urlBrand: Brand | null;
+  storedBrand: Brand | null;
+}): Brand {
+  if (urlBrand === SXWZ_BRAND) {
+    return SXWZ_BRAND;
+  }
+
+  if (urlBrand === DEFAULT_BRAND) {
+    return DEFAULT_BRAND;
+  }
+
+  if (storedBrand === SXWZ_BRAND) {
+    return SXWZ_BRAND;
+  }
+
+  return DEFAULT_BRAND;
+}
+
+export function getBrandRootClassName(brand: Brand): string {
+  return brand === SXWZ_BRAND ? "brand-sxwz" : "brand-default";
+}
+
+export function readStoredBrand(storage: Pick<Storage, "getItem">): Brand | null {
+  const value = storage.getItem(BRAND_SESSION_STORAGE_KEY);
+  return isBrand(value) ? value : null;
+}
+
+export function writeStoredBrand(
+  storage: Pick<Storage, "setItem">,
+  brand: Brand,
+): void {
+  storage.setItem(BRAND_SESSION_STORAGE_KEY, brand);
+}

frontend/src/core/brand/provider-client.tsx

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+"use client";
+
+import { useSearchParams } from "next/navigation";
+import { useLayoutEffect } from "react";
+
+import { useBrand } from "./provider";
+
+import {
+  parseBrandFromSearchParams,
+  readStoredBrand,
+  resolveBrandSession,
+  writeStoredBrand,
+} from "./index";
+
+export function BrandSessionInitializer() {
+  const searchParams = useSearchParams();
+  const { setBrand } = useBrand();
+
+  useLayoutEffect(() => {
+    const storedBrand = readStoredBrand(window.sessionStorage);
+    const urlBrand = parseBrandFromSearchParams(
+      new URLSearchParams(searchParams.toString()),
+    );
+    const resolvedBrand = resolveBrandSession({
+      urlBrand,
+      storedBrand,
+    });
+
+    writeStoredBrand(window.sessionStorage, resolvedBrand);
+    setBrand(resolvedBrand);
+  }, [searchParams, setBrand]);
+
+  return null;
+}

frontend/src/core/brand/provider.tsx

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+"use client";
+
+import { createContext, useContext, useState, type ReactNode } from "react";
+
+import {
+  BRAND_COPY,
+  DEFAULT_BRAND,
+  getBrandRootClassName,
+  type Brand,
+} from "./index";
+
+type BrandContextValue = {
+  brand: Brand;
+  copy: (typeof BRAND_COPY)[Brand];
+  rootClassName: string;
+  setBrand: (brand: Brand) => void;
+};
+
+const BrandContext = createContext<BrandContextValue | null>(null);
+
+function getInitialBrand(): Brand {
+  if (typeof window === "undefined") {
+    return DEFAULT_BRAND;
+  }
+
+  const storedBrand = window.sessionStorage.getItem("deerflow.brand-session");
+  return storedBrand === "sxwz" ? "sxwz" : DEFAULT_BRAND;
+}
+
+export function BrandProvider({ children }: { children: ReactNode }) {
+  const [brand, setBrand] = useState<Brand>(getInitialBrand);
+
+  return (
+    <BrandContext.Provider
+      value={{
+        brand,
+        copy: BRAND_COPY[brand],
+        rootClassName: getBrandRootClassName(brand),
+        setBrand,
+      }}
+    >
+      {children}
+    </BrandContext.Provider>
+  );
+}
+
+export function useBrand() {
+  const context = useContext(BrandContext);
+
+  if (!context) {
+    throw new Error("useBrand must be used within BrandProvider");
+  }
+
+  return context;
+}