修复消息首页会话预览恒为"暂无消息"，即使会话有真实历史消息

message-center-view.jsx 的预览文字读 topic.latestMessage()，这个方法
只会从话题自己的本地消息缓存里取值——而这个缓存只有在话题真正被
subscribe 过一次数据拉取后才会填充。me.contacts()（resetContactList）
给出的是每个联系人真实的、已缓存的Topic实例，包括用户这次会话里
从未打开过的——这些话题的latestMessage()恒为undefined，即使同一个
对象自己的touched/seq字段能证明服务端真实存在历史消息。

新增prefetchLatestMessagePreviews()：对chatList里seq>0但本地缓存为空
的话题，一次性拉取最新1条消息填充预览。用一个Set跟踪已尝试过的话题，
避免resetContactList()（几乎每个{meta}/{pres}事件都会触发一次）反复
重新拉取同一个话题。

调试过程中发现并修正了自己犯的两个错误：(1) resetContactList()在
websocket握手完成前就会被本地持久化缓存恢复触发一次，此时的subscribe
必然503失败，加了isConnected()判断跳过，避免过早失败把话题永久标记
为"已尝试"；(2) Topic.subscribe(getParams, setParams)的参数顺序搞反，
把查询对象传成了第二个参数(setParams)，导致subscribe成功但发出的
{sub}包完全没有get子句，从未真正请求过消息数据。

已用真实浏览器验证：好友kcg051307会话（真实71条历史消息）刷新页面
后预览从"暂无消息"变为真实的最新消息内容，未读角标数字不受影响
（确认没有误将未读消息标记为已读）。

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

src/views/tinode-web.jsx

@@ -216,6 +216,17 @@ class TinodeWeb extends React.Component {
 
     this.selfRef = React.createRef();
 
+    // message-center-view.jsx's conversation-list preview reads topic.latestMessage(), which the
+    // vendored SDK only ever populates from a topic's own local message cache -- itself only filled
+    // in once that specific topic has actually been subscribed with a data fetch. me.contacts()
+    // (resetContactList below) yields the real, already-cached Topic instances for every contact --
+    // including ones the user has never opened this session -- so most rows fall straight through to
+    // the '暂无消息' fallback despite `touched`/`seq` on the same object proving real history exists
+    // server-side. This tracks which topics we've already kicked off a one-shot "just the last
+    // message" fetch for, so repeated resetContactList() calls (it fires on nearly every {meta}/{pres}
+    // event, not just once at boot) don't re-subscribe the same topic over and over.
+    this.previewPrefetchAttempted = new Set();
+
     this.state = this.getBlankState();
 
     this.handleResize = this.handleResize.bind(this);
@@ -1284,6 +1295,8 @@ class TinodeWeb extends React.Component {
       }
     });
 
+    this.prefetchLatestMessagePreviews(newState.chatList);
+
     const past = new Date(0);
     newState.chatList.sort((a, b) => {
       // Pinned topics take precedence.
@@ -1297,6 +1310,41 @@ class TinodeWeb extends React.Component {
     this.setState(newState);
   }
 
+  // See the comment on this.previewPrefetchAttempted in the constructor for why this exists.
+  // seq > 0 is the same 'does this topic actually have any messages at all' signal
+  // message-center-view.jsx would otherwise have no way to distinguish from 'genuinely empty' --
+  // skip topics with no history instead of firing a subscribe that would just come back empty.
+  //
+  // !this.tinode.isConnected() live repro: resetContactList() fires as soon as the persisted local
+  // cache restores 'me's last-known contact list (log line "Persistent cache initialized"), which is
+  // *before* the websocket has even sent {hi} -- every subscribe() call made at that point instantly
+  // rejects with a 503 "Websocket is not connected". Skip attempting entirely while disconnected
+  // (rather than catching the failure) so these topics *aren't* added to previewPrefetchAttempted yet
+  // -- resetContactList() runs again moments later once the real {meta topic:'me'} response arrives
+  // over the now-live connection, and this gets a genuine second try then.
+  prefetchLatestMessagePreviews(chatList) {
+    if (!this.tinode.isConnected()) {
+      return;
+    }
+    for (const c of chatList) {
+      if (!c.topic || c.seq <= 0 || c.latestMessage() || this.previewPrefetchAttempted.has(c.topic)) {
+        continue;
+      }
+      this.previewPrefetchAttempted.add(c.topic);
+      // Topic.subscribe(getParams, setParams) -- the query is the *first* argument. Passing it
+      // second (as setParams) was silently swallowed: the resulting {sub} packet had no `get`
+      // clause at all (confirmed live: `out: {"sub":{"id":...,"topic":"..."}}`, no `get` key), so
+      // subscribe succeeded but never actually requested -- let alone received -- any message data.
+      c.subscribe(c.startMetaQuery().withLaterData(1).build()).catch(err => {
+        // Leave it in previewPrefetchAttempted either way -- a topic that fails once we're
+        // definitely connected (e.g. a stale contact the user no longer has access to) isn't going
+        // to succeed on the next resetContactList() a moment later either, and would otherwise retry
+        // on every single {meta}/{pres} event for the rest of the session.
+        console.warn(`Failed to prefetch latest message for '${c.topic}'`, err);
+      });
+    }
+  }
+
   static checkIfPinsUpdated(me, chatList) {
     for (let i = 0; i < chatList.length; i++) {
       const c = chatList[i];