feat(ui): unify list virtualization on @tanstack/react-virtual and polish scroll behavior

- Migrate sidebar session groups, git changes panel, virtualized code
  blocks, and JSON tree viewer from virtua to @tanstack/react-virtual;
  virtua remains only inside the Pierre diff viewer integration
- Sidebar: preserve scroll position when virtualization enables
  mid-session (enable only once the ancestor scroll element is resolved,
  seed initial offset from its live scrollTop, render plain rows for the
  single pre-paint frame); disable native scroll anchoring on the
  sessions scroller; keep row spacing identical between plain and
  virtualized modes; absolute row positioning so variable-height rows
  cannot drift past the container
- Chat: expand tool/thinking blocks downward by only adjusting scroll
  for rows growing above the viewport; raise the desktop history-load
  lead to 1.5 viewports so prepends land above the visible area
- Git changes: compute the prefetch window from the first visible row,
  skipping overscan rows above the viewport
- Sidebar rows: make the whole highlighted row area clickable, guarded
  against double-firing from interactive children

packages/ui/src/components/chat/message/parts/VirtualizedCodeBlock.tsx

@@ -3,7 +3,7 @@
  *
  * Renders large code/read outputs without mounting one highlighter per line:
  *   1. ONE worker tokenization of the whole block (off the main thread)
- *   2. virtua to only render visible rows
+ *   2. @tanstack/react-virtual to only render visible rows
  *
  * Tokenizing the whole block at once also preserves cross-line syntax context
  * (multi-line strings/comments) that per-line highlighting loses. Colors resolve
@@ -11,7 +11,7 @@
  */
 
 import React from 'react';
-import { Virtualizer } from 'virtua';
+import { useVirtualizer } from '@tanstack/react-virtual';
 import { useThemeSystem } from '@/contexts/useThemeSystem';
 import { getMarkdownSyntaxVars } from '@/components/chat/markdown/markdownTheme';
 import { useWorkerHighlightedLines } from '@/components/code/useWorkerHighlightedLines';
@@ -114,28 +114,41 @@ const VirtualizedRows: React.FC<VirtualizedRowsProps> = React.memo(({
   const parentRef = React.useRef<HTMLDivElement>(null);
   const viewportHeight = `min(${lines.length * ROW_HEIGHT}px, ${maxHeight})`;
 
+  const virtualizer = useVirtualizer<HTMLDivElement, HTMLDivElement>({
+    count: lines.length,
+    getScrollElement: () => parentRef.current,
+    estimateSize: () => ROW_HEIGHT,
+    overscan: 20,
+  });
+  const virtualItems = virtualizer.getVirtualItems();
+
   return (
     <div
       ref={parentRef}
       className="typography-code font-mono w-full min-w-0"
       style={{ ...(syntaxVars as React.CSSProperties), height: viewportHeight, maxHeight, overflow: 'auto' }}
     >
-      <Virtualizer
-        data={lines}
-        itemSize={ROW_HEIGHT}
-        bufferSize={ROW_HEIGHT * 20}
-        scrollRef={parentRef}
-      >
-        {(line, index) => (
-          <Row
-            key={index}
-            line={line}
-            html={highlighted?.[index]}
-            showLineNumbers={showLineNumbers}
-            style={lineStyles?.(line)}
-          />
-        )}
-      </Virtualizer>
+      <div style={{ height: virtualizer.getTotalSize(), position: 'relative' }}>
+        {virtualItems.map((item) => {
+          const line = lines[item.index];
+          if (!line) return null;
+          return (
+            <div
+              key={item.index}
+              data-index={item.index}
+              ref={virtualizer.measureElement}
+              style={{ position: 'absolute', top: 0, left: 0, width: '100%', transform: `translateY(${item.start}px)` }}
+            >
+              <Row
+                line={line}
+                html={highlighted?.[item.index]}
+                showLineNumbers={showLineNumbers}
+                style={lineStyles?.(line)}
+              />
+            </div>
+          );
+        })}
+      </div>
     </div>
   );
 });