fix(weixin): 扫码登录二维码真实可扫——前端生成QR(修坏图) + DecodeConfig容忍空config

weixin qrcode_img_content 是微信liteapp HTML页(非图片),后端fetch转base64只得<!doctype>→前端img坏图。
- 后端 login-qr 改返回 qrUrl(qrcode_img_content URL)+qrCode(原始qrcode),去掉坏 qrImage
- 前端用 qrcode 库从 qrUrl 生成真实二维码图片(微信扫https链接即开连接页), 新增 qrcode+@types/qrcode(纯UI依赖)
- DecodeConfig: nil config map 当空map解码(不报错), 否则 pending账号(空config)构建被跳过→未注册→登录端点404
- 测试更新(DecodeConfigNil/admin QR端点断言 qrUrl+qrCode)
- 另有 weixinAccountFor 裸weixin兜底(此前提交 d3b0921) 保证 accountId 命中
- 实测: login-qr 200{waiting, qrUrl=89}, 前端能看到可扫二维码
build/vet/test 全绿, 已部署.

frontend/web/src/admin/modules/channels/WeixinLogin.tsx

@@ -1,14 +1,37 @@
 import { useEffect, useRef, useState } from "react";
+import QRCode from "qrcode";
 import { adminWeixinLoginQR, adminWeixinLoginStatus } from "../../../lib/api";
 import { useAuditStore } from "../../audit";
 import { Button, Surface, Tag } from "../../../components/primitives";
 import { Notice } from "../../components";
 
 /** weixin-iLink scan-login panel (design/08 §4). On an account without a bot_token
- *  it shows a 扫码登录 button; clicking it mints a QR via GET /login-qr, renders the
- *  QR image, and polls GET /login-status until confirmation. On confirmation the
- *  backend persists the token and hot-connects the adapter, so the panel just
- *  flags "已登录". An account that already has a token never shows the button. */
+ *  it shows a 扫码登录 button; clicking it mints a QR via GET /login-qr, renders a
+ *  scannable QR from the WeChat QR URL (qrcode_img_content), and polls GET /login-status
+ *  until confirmation. The liteapp URL is an HTML page that cannot be fetched as an
+ *  image, so the frontend draws the QR itself from qrUrl. On confirmation the backend
+ *  persists the token and hot-connects the adapter; the panel flags "已登录". */
+function QrImage({ value }: { value: string }): React.ReactNode {
+  const [src, setSrc] = useState<string>("");
+  useEffect(() => {
+    let alive = true;
+    QRCode.toDataURL(value, { width: 200, margin: 1 })
+      .then((url) => alive && setSrc(url))
+      .catch(() => alive && setSrc(""));
+    return () => {
+      alive = false;
+    };
+  }, [value]);
+  if (!value) return <Notice tone="warn">未获得二维码，请点「刷新二维码」重试。</Notice>;
+  if (!src) return <Notice>生成二维码中…</Notice>;
+  return (
+    <img
+      src={src}
+      alt="微信 iLink 登录二维码"
+      className="h-48 w-48 rounded-[var(--mu-radius)] border border-[var(--mu-glass-border)] bg-[var(--mu-surface-2)] object-contain p-2"
+    />
+  );
+}
 export function WeixinLogin({
   platform,
   accountId,
@@ -18,7 +41,6 @@ export function WeixinLogin({
 }): React.ReactNode {
   const log = useAuditStore((s) => s.log);
   const [status, setStatus] = useState<string>("idle");
-  const [qrImage, setQrImage] = useState<string>("");
   const [qrUrl, setQrUrl] = useState<string>("");
   const [botId, setBotId] = useState<string>("");
   const [busy, setBusy] = useState<string | null>(null);
@@ -64,12 +86,10 @@ export function WeixinLogin({
   const start = () => {
     setBusy("__qr__");
     setNotice(null);
-    setQrImage("");
     setQrUrl("");
     adminWeixinLoginQR(platform, accountId)
       .then((r) => {
         log("channel.weixin.login.qr", "ok");
-        setQrImage(r.qrImage ?? "");
         setQrUrl(r.qrUrl ?? "");
         if (r.status === "confirmed") {
           setStatus("confirmed");
@@ -97,7 +117,7 @@ export function WeixinLogin({
       .finally(() => setBusy(null));
   };
 
-  const showQR = status === "waiting" || (qrImage !== "" || qrUrl !== "");
+  const showQR = status === "waiting" || qrUrl !== "";
 
   return (
     <Surface gap="sm" className="space-y-3">
@@ -124,14 +144,10 @@ export function WeixinLogin({
 
       {showQR ? (
         <div className="space-y-2">
-          {qrImage || qrUrl ? (
-            <img
-              src={qrImage || qrUrl}
-              alt="微信 iLink 登录二维码"
-              className="h-48 w-48 rounded-[var(--mu-radius)] border border-[var(--mu-glass-border)] bg-[var(--mu-surface-2)] object-contain p-2"
-            />
+          {qrUrl ? (
+            <QrImage value={qrUrl} />
           ) : (
-            <Notice tone="warn">二维码图片暂不可用，请点击「刷新二维码」重试。</Notice>
+            <Notice tone="warn">二维码暂不可用，请点击「刷新二维码」重试。</Notice>
           )}
           <p className="text-xs text-[var(--mu-text-lo)]">请用微信扫描上方二维码，确认后自动连接（最多等待约 8 分钟）。</p>
         </div>