切换到会话时输入框自动聚焦

匹配主流IM客户端的习惯——切进一个聊天窗口应该能直接开始打字，不需要先手动
点一下输入框。只在topicId变化时聚焦一次，不依赖messages（否则每来一条新
消息就会把正在做其他操作的用户的焦点抢走）。

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

web/src/features/messages/ConversationView.tsx

@@ -267,6 +267,15 @@ export function ConversationView() {
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [topicId]);
 
+  // Switching into a chat window should let the user start typing immediately, not require an extra
+  // click into the input first -- matches every mainstream IM client. Deliberately NOT keyed on
+  // `messages` (would re-steal focus away from whatever the user is doing every time a new message
+  // arrives while they're already composing) -- only the initial switch itself should grab focus.
+  useEffect(() => {
+    if (!topicId) return;
+    textareaRef.current?.focus();
+  }, [topicId]);
+
   // Someone else dissolving the group currently open here (or this account being removed from it)
   // fires the SDK's `onDeleteTopic`, not `onData`/`onPres` -- those only invalidate this screen's
   // cached snapshots for re-render, they don't tell it the topic itself is gone. Left unhandled, the