refactor: remove Subject field from Conversation model and related logic, replace with CustomerName

web/app/dashboard/conversations/_components/chat-panel.tsx

@@ -480,7 +480,7 @@ export function ChatPanel() {
             <DialogTitle>确认认领会话</DialogTitle>
             <DialogDescription>
               {conversation
-                ? `确认认领“${conversation.subject}”吗？认领后会话会进入我的列表。`
+                ? `确认认领“${conversation.customerName || `客户 #${conversation.customerId || conversation.id}`}”吗？认领后会话会进入我的列表。`
                 : "确认认领当前会话吗？"}
             </DialogDescription>
           </DialogHeader>

web/app/dashboard/conversations/_components/conversation-list.tsx

@@ -73,7 +73,7 @@ export function ConversationList({ onAfterSelect }: ConversationListProps) {
                     <div className="min-w-0 flex-1">
                       <div className="flex items-center gap-1.5">
                         <span className="min-w-0 flex-1 truncate font-medium text-sm leading-4">
-                          {conversation.subject}
+                          {conversation.customerName || `客户 #${conversation.customerId || conversation.id}`}
                         </span>
                         {conversation.agentUnreadCount > 0 ? (
                           <div className="flex size-4.5 shrink-0 items-center justify-center rounded-full bg-primary text-[10px] text-primary-foreground">