fix(导航): 关闭聊天/信息面板改为真实返回，不再新增历史记录

用户反馈：从会话列表打开聊天A，点页面自己的"返回"关掉聊天，再打开聊天B，这时候点
浏览器的返回键，会先回到列表，再点一次返回又跳回了聊天A——因为"关闭"这个动作和"打开"
一样都在往浏览器历史里push新记录，而不是弹出(pop)"打开"时push的那条，导致聊天A那条
记录一直遗留在历史栈里，多按一次返回就会被重新翻出来。

两处受影响、同一根因：
- tinode-web.jsx 的 handleHideMessagesView（移动端聊天页左上角"←"，messages-view.jsx
  的#hide-message-view）——打开聊天(handleTopicSelected)是真实push，关闭时原来无条件
  再push一条"没有话题"的状态，现在改成先尝试HashNavigation.goBack()，找不到可回退的
  历史才退回原来的push方式。
- info-view.jsx 的 handleBackNavigate（旧版InfoView自己的返回/关闭按钮，目前只有"已
  保存的消息"这一个话题还在用它，好友/群的资料页已经在更早的章节改用真正修过返回逻辑
  的friend-settings/group-settings）——同样的问题，同样的修法。

已在浏览器验证：连续"开聊天A→点返回关闭→开聊天B"后，浏览器返回键能一次性回到聊天A
从未存在过一样，不会再被重新翻出来；InfoView的关闭按钮点击后hash是异步变化的（真实
history.back()的特征），不再是原来同步的push。

src/views/tinode-web.jsx

@@ -1432,12 +1432,22 @@ class TinodeWeb extends React.Component {
     }
   }
 
-  // In mobile view user requested to show sidepanel
+  // In mobile view user requested to show sidepanel (this is the chat's own "<-" back arrow,
+  // #hide-message-view in messages-view.jsx). Opening a topic (handleTopicSelected) is a real
+  // navigateTo() push, so closing it should be the matching pop -- goBack() first, same pattern as
+  // content-library-view.jsx's Header back button. Before this fix it unconditionally pushed a new
+  // "no topic" hash entry instead of popping the "topic open" one: open chat A (push) -> tap back
+  // (push again, but the "chat A open" entry from a moment ago is left sitting right underneath,
+  // never consumed) -> open chat B (push) -> the system back button then has to be pressed twice to
+  // leave the chat area at all, and the second press lands back on chat A, not wherever the user
+  // actually came from before opening any chat.
   handleHideMessagesView() {
     this.setState({
       mobilePanel: 'sidepanel'
     });
-    HashNavigation.navigateTo(HashNavigation.setUrlTopic(window.location.hash, null));
+    if (!HashNavigation.goBack()) {
+      HashNavigation.navigateTo(HashNavigation.setUrlTopic(window.location.hash, null));
+    }
   }
 
   // User is sending a message, either plain text or a drafty object, possibly