fix(ui): use authenticated user avatarUrl instead of fallback initial in chat

src/app/app.html

@@ -143,7 +143,10 @@
               
               <!-- 用户头像：圆形 -->
               <div class="msg-avatar user-avatar" *ngIf="msg.role === 'user'">
-                {{ auth.user()?.name?.charAt(0) || '我' }}
+                <img *ngIf="auth.user()?.avatarUrl" [src]="auth.user()?.avatarUrl" alt="User" style="width: 100%; height: 100%; border-radius: 50%; object-fit: cover;">
+                <ng-container *ngIf="!auth.user()?.avatarUrl">
+                  {{ auth.user()?.name?.charAt(0) || '我' }}
+                </ng-container>
               </div>
             </div>
           }