optimize: WeChat QR code auth flow and channel reliability

- Generate QR code server-side when iLink API returns SPA page URL
  (added qrcode + image crates for PNG generation)
- Strip bot_token from frontend response (new WeixinQrcodeStatusPublic type)
- Add request timeouts and shared HTTP client for QR code endpoints
- Fix TOCTOU race on reply_context double-lock (single lock scope)
- Extract do_send() helper to deduplicate sendmessage logic; resend now
  checks ret field for context expiry instead of HTTP status only
- Cap pending_messages buffer at 50 to prevent unbounded memory growth
- Generate stable X-WECHAT-UIN per backend instance instead of per request
- Extract ILINK_CHANNEL_VERSION constant (was hardcoded in 4 places)
- Add 5-minute client-side QR expiry timeout in frontend dialog
- Track consecutive polling errors and show warning after 3 failures
- Stabilise onAuthSuccess/onClose callback refs to prevent polling restarts
- Replace dead i18n key weixinOpenQrcode with weixinPollError

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/i18n/messages/ar.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "انتهت صلاحية رمز QR.",
     "weixinRefreshQrcode": "تحديث",
     "weixinWaitingScan": "في انتظار المسح...",
-    "weixinOpenQrcode": "فتح رمز QR في المتصفح",
+    "weixinPollError": "الاتصال غير مستقر، جاري إعادة المحاولة...",
     "connect": "اتصال",
     "disconnect": "قطع الاتصال",
     "test": "اختبار الاتصال",

src/i18n/messages/de.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "QR-Code abgelaufen.",
     "weixinRefreshQrcode": "Aktualisieren",
     "weixinWaitingScan": "Warten auf Scan...",
-    "weixinOpenQrcode": "QR-Code im Browser öffnen",
+    "weixinPollError": "Verbindung instabil, erneuter Versuch...",
     "connect": "Verbinden",
     "disconnect": "Trennen",
     "test": "Verbindung testen",

src/i18n/messages/en.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "QR code expired.",
     "weixinRefreshQrcode": "Refresh",
     "weixinWaitingScan": "Waiting for scan...",
-    "weixinOpenQrcode": "Open QR code in browser",
+    "weixinPollError": "Connection unstable, retrying...",
     "connect": "Connect",
     "disconnect": "Disconnect",
     "test": "Test Connection",

src/i18n/messages/es.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "El código QR ha expirado.",
     "weixinRefreshQrcode": "Actualizar",
     "weixinWaitingScan": "Esperando escaneo...",
-    "weixinOpenQrcode": "Abrir código QR en el navegador",
+    "weixinPollError": "Conexión inestable, reintentando...",
     "connect": "Conectar",
     "disconnect": "Desconectar",
     "test": "Probar conexión",

src/i18n/messages/fr.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "QR code expiré.",
     "weixinRefreshQrcode": "Actualiser",
     "weixinWaitingScan": "En attente du scan...",
-    "weixinOpenQrcode": "Ouvrir le QR code dans le navigateur",
+    "weixinPollError": "Connexion instable, nouvelle tentative...",
     "connect": "Connecter",
     "disconnect": "Déconnecter",
     "test": "Tester la connexion",

src/i18n/messages/ja.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "QRコードの有効期限が切れました。",
     "weixinRefreshQrcode": "更新",
     "weixinWaitingScan": "スキャン待ち...",
-    "weixinOpenQrcode": "ブラウザでQRコードを開く",
+    "weixinPollError": "接続が不安定です。再試行中...",
     "connect": "接続",
     "disconnect": "切断",
     "test": "接続テスト",

src/i18n/messages/ko.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "QR 코드가 만료되었습니다.",
     "weixinRefreshQrcode": "새로고침",
     "weixinWaitingScan": "스캔 대기 중...",
-    "weixinOpenQrcode": "브라우저에서 QR 코드 열기",
+    "weixinPollError": "연결이 불안정합니다. 재시도 중...",
     "connect": "연결",
     "disconnect": "연결 해제",
     "test": "연결 테스트",

src/i18n/messages/pt.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "Código QR expirado.",
     "weixinRefreshQrcode": "Atualizar",
     "weixinWaitingScan": "Aguardando escaneamento...",
-    "weixinOpenQrcode": "Abrir código QR no navegador",
+    "weixinPollError": "Conexão instável, tentando novamente...",
     "connect": "Conectar",
     "disconnect": "Desconectar",
     "test": "Testar conexão",

src/i18n/messages/zh-CN.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "二维码已过期。",
     "weixinRefreshQrcode": "刷新二维码",
     "weixinWaitingScan": "等待扫码...",
-    "weixinOpenQrcode": "在浏览器中打开二维码",
+    "weixinPollError": "连接不稳定，正在重试...",
     "connect": "连接",
     "disconnect": "断开",
     "test": "测试连接",

src/i18n/messages/zh-TW.json

@@ -1719,7 +1719,7 @@
     "weixinQrcodeExpired": "二維碼已過期。",
     "weixinRefreshQrcode": "重新整理",
     "weixinWaitingScan": "等待掃碼...",
-    "weixinOpenQrcode": "在瀏覽器中打開二維碼",
+    "weixinPollError": "連接不穩定，正在重試...",
     "connect": "連線",
     "disconnect": "斷開",
     "test": "測試連線",