perf: skip off-screen ai-msg layout with content-visibility

Toggling the sidebar off/on with a large AI chat history (>1k messages,
>10k DOM nodes) causes ~750ms of synchronous reflow on show, because
`display: none` drops the layout object and Chromium rebuilds it from
scratch. Adding `content-visibility: auto` lets the browser skip layout
and paint for messages that are far off-screen; `contain-intrinsic-size`
supplies a height estimate so scroll geometry stays stable.

Measured on a 1098-message session: show-freeze dropped from ~752ms to
~89ms (~8.5x). Progressive enhancement — browsers without support just
ignore the property.

Author: abose

src/styles/Extn-AIChatPanel.less

@@ -270,6 +270,11 @@
 .ai-msg {
     margin-bottom: 12px;
     max-width: 100%;
+    // Skip layout / paint for messages that are far off-screen. For a large
+    // chat history (>1k messages, >10k DOM nodes), showing the sidebar after
+    // it was `display: none` otherwise costs ~750ms of synchronous reflow.
+    content-visibility: auto;
+    contain-intrinsic-size: auto 300px;
 
     .ai-msg-label {
         font-size: @sidebar-xs-font-size;