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/app/NavRail.tsx

@@ -1,5 +1,6 @@
 import { Link, useLocation } from 'react-router-dom';
 
+import { Avatar } from './Avatar';
 import { Icon } from './Icon';
 
 interface NavRailItem {
@@ -54,13 +55,20 @@ function NavRailButton({ item }: { item: NavRailItem }) {
   );
 }
 
+interface NavRailProps {
+  unreadMessages?: number;
+  /** The logged-in user's own id/name/avatar (`GET /v1/im/me`) -- drives the rail's top self-tile.
+   *  All optional so the rail still renders (as a letter-tile fallback) before that request settles. */
+  selfId?: string;
+  selfName?: string;
+  selfAvatarFileId?: string;
+}
+
 /** Persistent 64px icon rail, visible at >=640px (see App.tsx for the <640 bottom-tab fallback). */
-export function NavRail({ selfInitial = '我', unreadMessages = 0 }: { selfInitial?: string; unreadMessages?: number }) {
+export function NavRail({ unreadMessages = 0, selfId, selfName, selfAvatarFileId }: NavRailProps) {
   return (
     <nav className="flex h-full w-16 flex-none flex-col items-center border-r border-[#e6e9ee] bg-surface-rail pb-3 pt-3.5">
-      <div className="mb-[22px] grid h-[38px] w-[38px] place-items-center rounded-[10px] bg-accent text-base font-semibold text-white">
-        {selfInitial}
-      </div>
+      <Avatar id={selfId || ''} name={selfName} avatarFileId={selfAvatarFileId} className="mb-[22px] h-[38px] w-[38px] rounded-[10px] text-base" />
       <div className="flex flex-1 flex-col items-center gap-1.5">
         {TOP_ITEMS.map((item) => (
           <NavRailButton key={item.to} item={item.to === '/messages' ? { ...item, badge: unreadMessages } : item} />