feat: 敏感词管理页面（脱敏词后台化，限 lqq 账号）

- 后端 /api/sensitive-words：全局共享词表，开放读、admin 或 lqq 整体替换写
- 前端 desensitize 种子化：SEED_DESENSITIZE_MAP + 运行时 replaceDesensitizeMap
- 启动加载后台配置回灌脱敏映射；管理页支持增删改启停 + 一键导入种子词
- 入口置于左下角设置下拉（全量/简洁两种侧边栏均有），仅用户名 lqq 可见

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/src/App.tsx

@@ -2,6 +2,7 @@ import { useMemo } from "react";
 import { Navigate, Route, Routes } from "react-router-dom";
 import { ThemeProvider as NextThemesProvider } from "next-themes";
 
+import { DesensitizeWordsLoader } from "@/components/desensitize-words-loader";
 import { FontScaleApplier } from "@/components/font-scale";
 import { QueryClientProvider } from "@/components/query-client-provider";
 import { AuthSyncWatcher } from "@/core/auth/sync";
@@ -76,6 +77,7 @@ function AppProviders({ children }: { children: React.ReactNode }) {
           <ThemeProvider>
             <ChatProvider>
               <AuthSyncWatcher />
+              <DesensitizeWordsLoader />
               <FontScaleApplier />
               {children}
             </ChatProvider>