feat(web): 用符合系统风格的组件替换全部 window.alert/confirm

全项目搜出15处 window.confirm、9处 window.alert（其中3处是各文件
各自复制的 notImplemented() 辅助函数，文案还都一样）用在退出登录、
删除消息/收藏/好友、清空聊天记录、解散群聊、转让群主、危险链接
二次确认等场景——全是浏览器原生弹窗，样式和整个应用的设计语言完全
脱节。

新增两个全局UI基础设施（Zustand store + 挂载在App.tsx根节点的
渲染组件，参照stores/calls.ts/messageJump.ts已有的"任何组件都能调用"
惯例，不是每个弹窗各自管理一份本地状态）：

- stores/confirmDialog.ts + app/ConfirmDialogRoot.tsx：替换
  window.confirm。requestConfirm()返回Promise<boolean>，和
  window.confirm的返回值语义一致，所以原来"if (!window.confirm(...))
  return"这一行改成"if (!(await requestConfirm({message:...})))
  return"，外层函数标成async，不用大改周围逻辑。危险/不可恢复操作
  （删除、解散群聊、注销账号等）传danger:true，确认按钮变红色，
  弹窗视觉沿用转发弹窗已有的"描边取消+实心确认"按钮搭配。
- stores/globalToast.ts + app/GlobalToastRoot.tsx：替换window.alert，
  复用应用里本来就有、每个页面各自本地状态管理的Toast组件。顺手把
  3个文件里各自重复定义、文案完全一样的notImplemented()合并成
  一个共享导出。

实机验证了非危险确认（清空缓存，蓝色确认按钮）、危险确认（删除
消息，红色确认按钮）、取消按钮、Escape键取消、全局提示（点"投诉
举报"）——均正常工作，且原有页面各自的Toast错误提示不受影响。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/stores/confirmDialog.ts

@@ -0,0 +1,48 @@
+import { create } from 'zustand';
+
+/**
+ * Replaces `window.confirm()` app-wide (README's own design language never intended a native
+ * browser dialog to show up inside this UI -- it was just the fastest way to gate a destructive
+ * action while the real dialog didn't exist yet). A global store + a single `<ConfirmDialogRoot>`
+ * mounted once at the app root (see App.tsx), not a local `useState` per call site: this needs to be
+ * callable from any event handler, including ones with no natural "own a piece of JSX for this"
+ * home (e.g. GroupFilesModal's per-row delete button), the same reasoning `stores/calls.ts`/
+ * `stores/messageJump.ts` already established for other "any component, any time" actions.
+ *
+ * `requestConfirm()` returns a Promise the same shape `window.confirm()`'s return value has (resolves
+ * true/false, never rejects) so every existing `if (!window.confirm(...)) return;` call site becomes
+ * `if (!(await requestConfirm({ message: ... }))) return;` with the enclosing function marked async --
+ * a mechanical, low-risk edit, not a rewrite of the surrounding logic.
+ */
+export interface ConfirmOptions {
+  title?: string;
+  message: string;
+  confirmLabel?: string;
+  cancelLabel?: string;
+  /** Red confirm button for irreversible/destructive actions (delete, disband, deregister) --
+   *  omitted (default blue) for merely consequential ones (log out, switch tenant). */
+  danger?: boolean;
+}
+
+interface PendingConfirm extends ConfirmOptions {
+  resolve: (value: boolean) => void;
+}
+
+interface ConfirmDialogState {
+  pending: PendingConfirm | null;
+  respond: (value: boolean) => void;
+}
+
+export const useConfirmDialogStore = create<ConfirmDialogState>((set, get) => ({
+  pending: null,
+  respond: (value) => {
+    get().pending?.resolve(value);
+    set({ pending: null });
+  },
+}));
+
+export function requestConfirm(options: ConfirmOptions): Promise<boolean> {
+  return new Promise((resolve) => {
+    useConfirmDialogStore.setState({ pending: { ...options, resolve } });
+  });
+}

web/src/stores/globalToast.ts

@@ -0,0 +1,37 @@
+import { create } from 'zustand';
+
+/**
+ * Replaces `window.alert()` app-wide, same rationale as `stores/confirmDialog.ts`'s replacement of
+ * `window.confirm()`: a global store + a single `<GlobalToastRoot>` mounted once at the app root
+ * renders the app's own existing `Toast` component (already used everywhere for a mutation's error
+ * state) instead of the browser's native dialog. `notify()` is deliberately fire-and-forget (no
+ * Promise, no return value) -- an alert-style message never blocked on a user response the way
+ * confirm did, so there's nothing to await.
+ */
+interface GlobalToastState {
+  message: string;
+  variant: 'success' | 'error';
+  token: number;
+}
+
+export const useGlobalToastStore = create<GlobalToastState>(() => ({
+  message: '',
+  variant: 'error',
+  token: 0,
+}));
+
+export function notify(message: string, variant: 'success' | 'error' = 'error') {
+  // `token` (not just `message`) is what Toast's own auto-dismiss effect keys off of via the caller
+  // -- see GlobalToastRoot -- so firing the SAME message twice in a row (e.g. two clicks on a
+  // still-disabled "该功能暂未开放" button before the first toast dismisses) restarts the timer
+  // instead of the second call being a no-op change that React skips re-rendering for.
+  useGlobalToastStore.setState((s) => ({ message, variant, token: s.token + 1 }));
+}
+
+/** Every existing `window.alert('该功能暂未开放，如需联系客服。')` call site (settings, composer
+ *  toolbar, add-friend, group/contact detail panels) used the identical hand-typed Chinese string --
+ *  one shared helper instead of that string living independently in five files that could each drift
+ *  from the others if the wording ever needs to change. */
+export function notImplemented() {
+  notify('该功能暂未开放，如需联系客服。');
+}