修复通话发起后立即退出登录导致应用整体白屏崩溃

handleLogout()先断开WS连接、再重置界面状态，界面重置卸载
MessagesView时其componentWillUnmount会尝试发一条挂断信令，vendored
SDK的Connection.sendText()在已关闭的socket上同步抛错，componentWillUnmount
里没有error boundary兜底，导致整个应用被React卸载成白屏。

5处几乎相同的通话信令发送方法（ringing/hang-up/offer/ice-candidate/
answer）合并成一个sendCallSignal()辅助方法，发送前检查连接状态、
发送本身包一层try/catch。

第十二轮最小粒度测试 Ch8 发现（点击通话按钮后立即退出登录复现）。

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

src/views/tinode-web.jsx

@@ -2511,48 +2511,56 @@ class TinodeWeb extends React.Component {
     }
   }
 
-  handleCallRinging(callTopic, callSeq) {
+  // All of the call-signaling senders below (ringing/hang-up/offer/ice-candidate/answer) go through
+  // this one helper rather than calling topic.videoCall() directly. handleCallHangup in particular is
+  // called from several unmount/teardown paths outside the normal call flow (messages-view.jsx's
+  // componentWillUnmount and its topic-switch handler in componentSetup both call it via the
+  // onCallHangup prop, on top of the ring-timeout path and CallInvite's own cleanup here) -- including
+  // ones that fire *after* a logout has already torn down the connection: handleLogout() calls
+  // this.tinode.disconnect() and only afterwards resets state to the blank/logged-out screen, and that
+  // state reset is what unmounts messages-view.jsx and trips its componentWillUnmount -> handleCallHangup
+  // call. Real repro: click a call button (real or scripted handleStartVideoCall/handleStartAudioCall),
+  // then immediately log out -- topic.videoCall() ends up calling the vendored SDK's
+  // Connection.sendText() on an already-closed socket, which throws synchronously ("Websocket is not
+  // connected"). Thrown from componentWillUnmount, React has no error boundary for it and unmounts the
+  // entire app to a blank white page. None of these are anything the peer can still receive over a dead
+  // connection anyway (and for hang-up specifically, the server's own establishment-timeout already
+  // cleans up an abandoned call-in-progress independently), so skip sending entirely once disconnected
+  // instead of letting it throw -- the other four signals are currently unreachable in practice (no real
+  // call can get established while "call invite always 400s" -- see handleCallInvite -- is unfixed) but
+  // share the identical topic.videoCall() shape and would hit the exact same crash the moment that's
+  // fixed and real in-progress calls (offer/ice-candidate/answer exchange, or a mid-call logout) become
+  // possible.
+  sendCallSignal(callTopic, evt, callSeq, payload) {
     const topic = this.tinode.getTopic(callTopic);
-    if (!topic) {
+    if (!topic || !this.tinode.isConnected()) {
       return;
     }
+    try {
+      topic.videoCall(evt, callSeq, payload);
+    } catch (err) {
+      console.warn(`Failed to send call signal '${evt}'`, err);
+    }
+  }
 
-    topic.videoCall('ringing', callSeq);
+  handleCallRinging(callTopic, callSeq) {
+    this.sendCallSignal(callTopic, 'ringing', callSeq);
   }
 
   handleCallHangup(callTopic, callSeq) {
-    const topic = this.tinode.getTopic(callTopic);
+    this.sendCallSignal(callTopic, 'hang-up', callSeq);
-    if (!topic) {
-      return;
-    }
-
-    topic.videoCall('hang-up', callSeq);
   }
 
   handleCallSendOffer(callTopic, callSeq, sdp) {
-    const topic = this.tinode.getTopic(callTopic);
+    this.sendCallSignal(callTopic, 'offer', callSeq, sdp);
-    if (!topic) {
-      return;
-    }
-
-    topic.videoCall('offer', callSeq, sdp);
   }
 
   handleCallIceCandidate(callTopic, callSeq, candidate) {
-    const topic = this.tinode.getTopic(callTopic);
+    this.sendCallSignal(callTopic, 'ice-candidate', callSeq, candidate);
-    if (!topic) {
-      return;
-    }
-
-    topic.videoCall('ice-candidate', callSeq, candidate);
   }
-  handleCallSendAnswer(callTopic, callSeq, sdp) {
-    const topic = this.tinode.getTopic(callTopic);
-    if (!topic) {
-      return;
-    }
 
-    topic.videoCall('answer', callSeq, sdp);
+  handleCallSendAnswer(callTopic, callSeq, sdp) {
+    this.sendCallSignal(callTopic, 'answer', callSeq, sdp);
   }
 
   handleCallClose() {