perf: CSS content-visibility + containment 优化长对话渲染性能

- .message-block 添加 content-visibility: auto 跳过屏幕外消息渲染
- .messages-flow / .messages-area 添加 contain: layout style 隔离布局
- 减少长对话 DOM 重排开销，解决横向 resize 时的级联卡顿

static/src/styles/components/chat/_chat-area.scss

@@ -100,6 +100,7 @@
   padding-bottom: 20px;
   min-height: 0;
   position: relative;
+  contain: layout style;
 }
 
 .messages-area.messages-area--stick {
@@ -113,6 +114,7 @@
   margin: 0 auto;
   display: flex;
   flex-direction: column;
+  contain: layout style;
 }
 
 .messages-bottom-spacer {
@@ -451,6 +453,8 @@
 .message-block {
   margin-bottom: 24px;
   min-width: 0;
+  content-visibility: auto;
+  contain-intrinsic-size: auto 300px;
 }
 
 .message-block.message-block--sub-agent-notice {