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/session/sidebar/SessionNodeItem.tsx

@@ -747,6 +747,18 @@ function SessionNodeItemComponent(props: Props): React.ReactNode {
     handleSessionSelect(session.id, sessionDirectory, projectId);
   };
 
+  // The selection/active highlight covers the WHOLE row box (gutter, edge
+  // paddings), while the primary click target is the inner title button.
+  // Make the rest of the highlighted box clickable too — but only for clicks
+  // that did not originate from an interactive child (title button, chevron,
+  // action menu), so nothing double-fires.
+  const handleRowBackgroundClick = (event: React.MouseEvent<HTMLDivElement>) => {
+    if (event.defaultPrevented) return;
+    const target = event.target as HTMLElement | null;
+    if (target?.closest('button, a, input, [role="menuitem"], [role="menu"]')) return;
+    handleRowSelect(event as unknown as React.MouseEvent<HTMLButtonElement>);
+  };
+
   const handleRowMouseDown = (event: React.MouseEvent<HTMLButtonElement>) => {
     if (event.button === 2 || (event.button === 0 && event.ctrlKey && !selectionModeEnabled)) {
       suppressNextSelectRef.current = true;
@@ -974,8 +986,9 @@ function SessionNodeItemComponent(props: Props): React.ReactNode {
                 data-session-row={session.id}
                 data-session-scope={sessionDirectory ?? ''}
                 data-session-archived={archivedBucket ? '1' : '0'}
+                onClick={handleRowBackgroundClick}
                 className={cn(
-                  'group relative my-0.5 flex items-center rounded-md py-1 pr-1.5',
+                  'group relative my-0.5 flex cursor-pointer items-center rounded-md py-1 pr-1.5',
                   // Pull the row box left into the container gutter so the
                   // selection highlight covers the chevron/status markers
                   // (which sit in that gutter), then re-pad so the title text