perf(ui): virtualize message timeline rendering, #274 follow-up ( BIG SPEED IMPROVEMENT ) (#291)

## Summary
- virtualize MessageTimeline so large session histories stop rendering
the full timeline sidebar at once.
- keep the existing full render path in selection mode so xray/selection
behavior stays intact.
- route active-segment scrolling through the virtualizer so timeline
navigation still follows the selected message.

## Benefit
- prompt field was very laggy in cession with big history and timeline
had many bugs, this is fixed.
- the session with big history now load as fast as a new session .

packages/ui/src/styles/messaging/message-timeline.css

@@ -66,10 +66,11 @@
 }
 
 .message-timeline {
+  --message-timeline-segment-gap: 0.35rem;
   flex: 1 1 auto;
   display: flex;
   flex-direction: column;
-  gap: 0.35rem;
+  gap: 0;
   padding: 0.25rem;
   overflow-y: auto;
   overflow-x: visible;
@@ -114,6 +115,17 @@
   -webkit-touch-callout: none;
 }
 
+.message-timeline-item {
+  display: flex;
+  flex-direction: column;
+  width: 100%;
+}
+
+.message-timeline-item-spacer {
+  flex: none;
+  width: 100%;
+}
+
 .message-timeline-segment[data-delete-hover="true"]::before {
   content: "";
   position: absolute;
@@ -319,18 +331,7 @@
   border-inline-start: 3px solid color-mix(in oklab, var(--accent-primary) 35%, transparent);
 }
 
-/* Extra spacing before the first tool in a group to separate from the
-   preceding user/assistant badge. */
-.message-timeline-group-start {
-  margin-top: 0.35rem;
-}
-
-/* Subtle extra spacing after the group parent (assistant) to separate
-   from the next user badge below. Uses adjacent sibling targeting. */
-.message-timeline-group-parent + .message-timeline-user,
-.message-timeline-group-parent + .message-timeline-compaction {
-  margin-top: 0.35rem;
-}
+/* Spacing is rendered by the measured item wrapper so virtua can account for it. */
 
 .message-timeline-container {
   position: relative;