feat(admin): 渠道状态三态化 + 顶栏后端失联指示 — 不再把「读不到」画成「未连接」

起因：面板把「取数失败（后端不可达）」渲染成红色「未连接」，主人因此把「后端没在跑」误读成「微信适配器挂了」（AdapterDetailPage 的 record?.connected ?? false）。
- 新增 channels/channelStatus.ts（纯逻辑三态：ready/pending/unknown + 记录级 connected/disconnected/unconfigured/unknown/pending；局部类型，未动 lib/types.ts）
- 新增 channelStatusView.tsx：ChannelStatusTag（三态 Tag + 「数据可能过期」）+ ChannelLoadError（失败/过期横幅 + 重试）
- 覆盖 13 处渲染点（channels/ 全目录 + AdminApp）：未知态一律灰色「状态未知」+ 原因，绝不落红色「未连接」；未知态禁用连接/断开与「新建/编辑配置」（避免 prefill 空列表误建/覆盖）；读取失败不再冒充「尚未保存任何渠道配置」「暂无消息」「未声明能力」等事实性谎言
- 顺带修：WeixinLogin 首次 login-status 读取失败被 .catch 静默吞成「未登录」，现如实报「读取失败（与后端失联）」
- AdminApp 顶栏新增 useBackendHealth + BackendStatusTag：HTTP /api/v1/health 探活（15s 循环），**探测成功前不显示「在线」**；WS 状态作为独立 Tag 展示，两者不混算
- 验收：npm run build 绿（177 模块）+ 新增 45 断言 + 既有 54 断言回归 + renderToStaticMarkup 真渲染冒烟（unknown 渲染不含「未连接」也不含 err 色；未探测成功不显示在线）
- ListPage 主函数 125→71 行（改前已超 80 行红线，行为等价）

frontend/web/src/admin/AdminApp.tsx

@@ -1,9 +1,11 @@
-import { useState } from "react";
+import { useEffect, useState } from "react";
 import { applyTheme, readTheme, type ThemeMode } from "../lib/theme";
-import { getAuthToken, getStoredRole } from "../lib/api";
+import { getAuthToken, getStoredRole, health } from "../lib/api";
+import { useAppStore } from "../store/useAppStore";
+import type { WsStatus } from "../lib/types";
 import { ADMIN_GROUPS, ADMIN_SECTIONS } from "./sections";
 import { navigateAdmin, type AdminSectionId } from "./router";
-import { Button, Tag } from "../components/primitives";
+import { Button, Tag, type TagTone } from "../components/primitives";
 import { Icon } from "../components/Icon";
 import { DashboardModule } from "./modules/Dashboard";
 import { PersonasModule } from "./modules/Personas";
@@ -117,6 +119,77 @@ function AdminMobileNav({ open, active, onClose }: { open: boolean; active: Admi
   );
 }
 
+/** Poll interval for the backend liveness probe (GET /api/v1/health). */
+const HEALTH_POLL_MS = 15_000;
+
+type BackendState = "checking" | "online" | "offline";
+
+const BACKEND_META: Record<BackendState, { label: string; tone: TagTone; dot: string }> = {
+  checking: { label: "后端检测中", tone: "neutral", dot: "mu-blink" },
+  online: { label: "后端在线", tone: "ok", dot: "bg-[var(--mu-ok)]" },
+  offline: { label: "后端失联", tone: "err", dot: "bg-[var(--mu-err)]" },
+};
+
+const WS_META: Record<WsStatus, { label: string; tone: TagTone; dot: string }> = {
+  idle: { label: "未连接", tone: "neutral", dot: "bg-[var(--mu-text-lo)]" },
+  connecting: { label: "连接中", tone: "warn", dot: "mu-blink" },
+  connected: { label: "实时", tone: "ok", dot: "bg-[var(--mu-ok)]" },
+  reconnecting: { label: "重连中", tone: "warn", dot: "mu-blink" },
+  disconnected: { label: "已断开", tone: "err", dot: "bg-[var(--mu-err)]" },
+  expired: { label: "登录过期", tone: "err", dot: "bg-[var(--mu-err)]" },
+};
+
+/** Real liveness probe — the tag must never claim 在线 before /health has actually
+ *  answered, so it starts as "检测中" (which is a state of its own, not a guess).
+ *  The realtime channel is a separate signal from the store: it can be down while
+ *  REST is fine (and vice versa), so it is shown as its own tag. */
+function useBackendHealth(): BackendState {
+  const [state, setState] = useState<BackendState>("checking");
+  useEffect(() => {
+    let cancelled = false;
+    const probe = () => {
+      health()
+        .then(() => {
+          if (!cancelled) setState("online");
+        })
+        .catch(() => {
+          if (!cancelled) setState("offline");
+        });
+    };
+    probe();
+    const id = window.setInterval(probe, HEALTH_POLL_MS);
+    return () => {
+      cancelled = true;
+      window.clearInterval(id);
+    };
+  }, []);
+  return state;
+}
+
+/** 后端在线 / 失联 + 实时通道状态, both from real signals. */
+function BackendStatusTag(): React.ReactNode {
+  const backend = useBackendHealth();
+  const wsStatus = useAppStore((s) => s.wsStatus);
+  const meta = BACKEND_META[backend];
+  const ws = WS_META[wsStatus];
+  return (
+    <>
+      <span className="inline-flex shrink-0" title={`GET /api/v1/health · 每 ${HEALTH_POLL_MS / 1000}s 探测一次`}>
+        <Tag tone={meta.tone}>
+          <span className={`inline-block h-1.5 w-1.5 rounded-full ${meta.dot}`} />
+          {meta.label}
+        </Tag>
+      </span>
+      <span className="hidden shrink-0 sm:inline-flex" title="实时通道（/ws）状态">
+        <Tag tone={ws.tone}>
+          <span className={`inline-block h-1.5 w-1.5 rounded-full ${ws.dot}`} />
+          实时通道 · {ws.label}
+        </Tag>
+      </span>
+    </>
+  );
+}
+
 function AdminTopBar({ section, onMenu }: { section: AdminSectionId; onMenu?: () => void }): React.ReactNode {
   const role = getStoredRole();
   const [theme, setTheme] = useState<ThemeMode>(readTheme);
@@ -152,6 +225,7 @@ function AdminTopBar({ section, onMenu }: { section: AdminSectionId; onMenu?: ()
           {role}
         </Tag>
       ) : null}
+      <BackendStatusTag />
       <button
         onClick={toggleTheme}
         aria-label="切换主题"