修复发送含链接消息时切换话题，风控弹窗可能把消息送错会话

现象：在会话A输入含链接的消息并点发送，POST /security/urls/check 请求还未返回时切换到
会话B，请求返回后风控弹窗会在会话B里弹出（本该属于会话A），此时点"继续发送"，消息被
真实发进了会话B，而不是最初输入它的会话A——跨会话消息误投递。

复现：会话A（kcg051307）输入"https://example.com/racefix?token=fix002"点发送，立即
（在checkURL网络请求返回前）切换到会话B（r12c56b2），风控弹窗在会话B视图下弹出；点
"继续发送"后消息被投递进会话B，会话A完全没有收到，会话B收到了本不该发给它的内容
（本次因B恰好还不是好友触发"发送失败"提示，但发送动作本身已经指向了错误的会话，
如果B是好友会真实送达）。

根因：`getDerivedStateFromProps`在话题切换时会把`riskDialog`重置为null，但这只能防住
"弹窗已经弹出、随后切换话题"这一种时序——它防不住"checkURL还在进行中就切换话题，
此时机的.then()回调在切换之后才执行"这种反过来的时序：这次setState({riskDialog:{...}})
是在切换完成之后才发生的，不会被这次重置覆盖，于是在当前（错误的）话题下重新弹出弹窗，
且弹窗里的`doSend`闭包沿用的是`this.props.sendMessage`，按确认时的当前话题发送，
不是消息最初输入时所在的话题。

修法：sendMessage()进入URL风险检测分支时已经缓存了`topicForDraft`（用于清草稿），
在checkURL的.then()回调最开头加一道判断——如果`this.state.topic`已经不等于
`topicForDraft`，说明用户已经离开发起这次检测的会话，直接返回false（视同消息被放弃，
不弹窗、不发送），不再继续读取风控结果或setState。这与本文件已有的"两条风险消息连续
触发、后一个覆盖前一个"场景处理逻辑（resolve(false)后覆盖）是同一种"宁可丢弃، 不可
错误投递"的一贯取舍，只是这次要防的是切换会话而不是重复触发。

验证：npm run build:dev后浏览器实测——(1) 会话A输入风险链接点发送，立即切换到会话B，
2秒后确认会话B里没有弹出风控弹窗（修复前必现）；(2) 切回会话A确认最后一条消息仍是修复
前测试遗留的旧消息，本次新消息没有出现在A也没有出现在B，两个会话都没有收到，符合
"宁可丢弃不可错投"的设计；(3) 对照组（不切换话题、正常触发风控）依旧能看到弹窗并正确
发送/取消，未被这次改动影响。

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

src/views/messages-view.jsx

@@ -1654,6 +1654,23 @@ class MessagesView extends React.Component {
       const RISK_RANK = {block: 2, warn: 1, allow: 0};
       const urls = Array.from(new Set(urlMatches));
       return Promise.all(urls.map(url => checkURL(url, 'message_compose'))).then(results => {
+        // The user may have already switched to a different conversation while checkURL was
+        // still in flight. getDerivedStateFromProps only protects against an ALREADY-shown
+        // dialog surviving a later topic switch (it nulls riskDialog when nextProps.topic !=
+        // prevState.topic) -- it can't protect against this .then() running AFTER that switch,
+        // which would otherwise setState({riskDialog:{...}}) fresh right here, popping the risk
+        // dialog up while a completely different conversation is now open. doSend() still calls
+        // this.props.sendMessage using whatever topic is current AT CONFIRM TIME (see doSend's
+        // own comment above), not the one this message was actually typed in, so confirming that
+        // dialog would silently deliver this message into the wrong conversation. Confirmed live:
+        // typed a flagged URL in topic A, switched to topic B before checkURL resolved, and the
+        // resulting dialog rendered while viewing topic B -- confirming it delivered topic A's
+        // message into topic B instead. If the topic has moved on, treat this message the same
+        // as if it were abandoned -- no dialog, no send -- the same "lost, but not hung forever"
+        // outcome already accepted just below for two flagged messages sent in quick succession.
+        if (this.state.topic !== topicForDraft) {
+          return false;
+        }
         const result = results.reduce((worst, r) => (RISK_RANK[r.decision] || 0) > (RISK_RANK[worst.decision] || 0) ? r : worst);
         if (result.decision === 'allow') {
           return doSend();