fix(frontend): 聊天页单屏布局并消除 F5 滚底闪烁

锁定视口避免双滚动；消息列表改用 flex-col-reverse，SSR/无 JS 默认贴最新消息。

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/app/chat/ChatShell.tsx

@@ -290,10 +290,8 @@ export default function ChatShell({
 
   return (
     <div
-      className="panel overflow-hidden grid grid-cols-1 lg:grid-cols-[320px_minmax(0,1fr)]"
+      className="panel overflow-hidden grid grid-cols-1 lg:grid-cols-[320px_minmax(0,1fr)] flex-1 min-h-0"
       style={{
-        height: "calc(100vh - 8.5rem)",
-        minHeight: "480px",
         borderColor: "var(--line)",
       }}
     >