fix: propagate real message IDs via SSE and fix regenerate model parameter

- Add message_id and user_message_id to SSE done events so frontend
  can replace temp IDs with real DB IDs, fixing "消息不存在" on regenerate
- Replace Body(default=None) with RegenerateRequest Pydantic model for
  proper JSON body parsing, fixing 422 Unprocessable Content
- Frontend always sends model in regenerate request body
- Pass selected model through message-item → regenerateMessage chain
- Various UI refinements to chat sidebar, quick questions, and layout

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/components/chat/message-list.tsx

@@ -5,13 +5,14 @@ import MessageItem from "./message-item";
 
 interface MessageListProps {
   messages: ChatMessage[];
+  selectedModel?: string;
 }
 
-export default function MessageList({ messages }: MessageListProps) {
+export default function MessageList({ messages, selectedModel }: MessageListProps) {
   return (
-    <div className="space-y-4 p-4">
+    <div className="max-w-3xl mx-auto px-4">
       {messages.map((message) => (
-        <MessageItem key={message.id} message={message} />
+        <MessageItem key={message.id} message={message} selectedModel={selectedModel} />
       ))}
     </div>
   );