fix(web): 左上角自己头像接入真实数据，会话订阅失败不再静默吞错

NavRail 的自己头像此前是硬编码的"我"字，AppShell 从没往下传过真实的
user_id/nickname/avatar_file_id，所以无论是谁登录、有没有上传头像都不会变；
现在改为拉取 /v1/im/me 并用 Avatar 组件渲染，和设置页/联系人等处一致。

ConversationView 打开会话失败时原来是 .catch(() => {}) 整个吞掉，用户会看到
空白的消息列表且没有任何提示；由于已读回执只在拿到消息后才会发，未读角标
也会因此永久卡住。现在失败会弹出错误提示，不再静默。

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

web/src/features/messages/ConversationView.tsx

@@ -129,10 +129,16 @@ export function ConversationView() {
 
   useEffect(() => {
     if (!topicId) return undefined;
-    openConversation(topicId).catch(() => {});
+    // Unlike every other action in this component (see showError's own comment above), a failed
+    // subscribe here used to just vanish into an empty `.catch(() => {})` -- the user would land on
+    // a totally blank message list with no error, no retry, and (since the noteRead effect below
+    // only fires once `messages.length > 0`) a read receipt that can never be sent, leaving the chat
+    // list's unread badge permanently stuck for a conversation that in fact never loaded at all.
+    openConversation(topicId).catch((err) => showError(err, '加载会话失败，请重试'));
     return () => {
       closeConversation(topicId).catch(() => {});
     };
+    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [topicId]);
 
   // useLayoutEffect, not useEffect -- this component never remounts on topicId change (route is