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/globals.css

@@ -113,6 +113,26 @@ body {
   text-rendering: optimizeLegibility;
 }
 
+/* 聊天页：整页锁在视口内，避免「页面滚动 + 对话区滚动」双滚动条 */
+body:has([data-chat-viewport]) {
+  height: 100dvh;
+  max-height: 100dvh;
+  overflow: hidden;
+}
+body:has([data-chat-viewport]) > footer {
+  display: none;
+}
+body:has([data-chat-viewport]) #main {
+  flex: 1 1 0;
+  min-height: 0;
+  display: flex;
+  flex-direction: column;
+  /* 覆盖 root layout 的 py-7/sm:py-9，只留紧凑间距 */
+  padding-top: 0.75rem;
+  padding-bottom: 0.75rem;
+  overflow: hidden;
+}
+
 ::selection {
   background: var(--accent);
   color: var(--accent-on);