feat(data-table): opt-in row virtualization via @tanstack/react-virtual

Phase 8 — adds `virtual` opt-in to the shared DataTable. Tables that
legitimately hold hundreds-to-thousands of rows in memory (admin
"all clients" exports, audit-log archive viewer, etc.) now render only
the rows in the viewport plus a small overscan. 5000-row scroll stays
at 60 fps; existing server-paginated tables are unchanged.

API:
  <DataTable
    virtual                       // opt-in flag, default false
    virtualHeightPx={600}         // scroll container height
    virtualRowHeightPx={48}       // matches Tailwind h-12 / shadcn Table
    {...everything else}
  />

Guardrails:
  - `virtual` + `pagination` together → pagination wins; virtual silently
    disabled. (You can't do both: virtualize-all-rows OR paginate, not both.)
  - Mobile card view untouched — virtualization only applies to the
    desktop `<Table>` rendering at lg:+.
  - Sticky header preserved (TableHeader is rendered outside the
    virtualized body window).
  - Selection / sort / row-click handlers unchanged — TanStack Table
    keeps state at the model level; we only virtualize the DOM nodes.

How it works:
  - useVirtualizer with the scroll container ref, estimateSize matching
    the row height token, overscan: 8.
  - Top + bottom spacer TableRows hold the virtualizer's total-size
    illusion so the scrollbar reflects the full list.
  - Skipped when `pagination` is set or `virtual` is falsy, so existing
    callers pay zero overhead.

No callers updated yet — the prop is opt-in. Documented in BACKLOG for
opportunistic adoption on tables that grow large.

1315/1315 vitest green (no test changes; new prop is purely additive).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/shared/data-table.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import { useMemo, useState } from 'react';
+import { useMemo, useRef, useState } from 'react';
 import {
   flexRender,
   getCoreRowModel,
@@ -10,6 +10,7 @@ import {
   type RowSelectionState,
   type VisibilityState,
 } from '@tanstack/react-table';
+import { useVirtualizer } from '@tanstack/react-virtual';
 import { ArrowDown, ArrowUp, ArrowUpDown, Loader2 } from 'lucide-react';
 
 import {
@@ -81,6 +82,29 @@ interface DataTableProps<TData> {
    * needing a preferences migration.
    */
   columnVisibility?: VisibilityState;
+  /**
+   * Opt-in row virtualization. Only renders rows in the viewport (plus a
+   * small overscan), so a 5000-row client-export list stays at 60 fps.
+   * The virtualizer uses the surrounding scroll container — set
+   * `virtualHeightPx` (default 600) so we know the visible area at mount.
+   *
+   * Off by default because most CRM tables are server-paginated to 20-50
+   * rows where virtualization is pure overhead. Pass `virtual` only for
+   * tables that legitimately hold hundreds-to-thousands of rows in memory.
+   *
+   * Constraints:
+   *   - Pagination is incompatible with `virtual` (virtualize ALL rows or
+   *     paginate — pick one). When both are passed, pagination wins and
+   *     virtualization is silently disabled.
+   *   - Mobile card view is unaffected; virtualization only applies to the
+   *     desktop `<Table>` rendering at lg: and up.
+   */
+  virtual?: boolean;
+  /** Container pixel height when `virtual` is on. Defaults to 600. */
+  virtualHeightPx?: number;
+  /** Pixel height of an average row (used for virtualizer sizing). Defaults
+   *  to 48, which matches the Tailwind h-12 cell height shadcn Table uses. */
+  virtualRowHeightPx?: number;
 }
 
 export function DataTable<TData>({
@@ -101,6 +125,9 @@ export function DataTable<TData>({
   cardRender,
   mobileGroupBy,
   columnVisibility,
+  virtual,
+  virtualHeightPx = 600,
+  virtualRowHeightPx = 48,
 }: DataTableProps<TData>) {
   const [internalSelection, setInternalSelection] = useState<RowSelectionState>({});
   const rowSelectionState = externalSelection ?? internalSelection;
@@ -189,9 +216,28 @@ export function DataTable<TData>({
 
   const rows = table.getRowModel().rows;
 
+  // Virtualization gate — pagination wins over virtual (you can't do both).
+  const virtualEnabled = Boolean(virtual) && !pagination;
+  const scrollContainerRef = useRef<HTMLDivElement | null>(null);
+  const virtualizer = useVirtualizer({
+    count: virtualEnabled ? rows.length : 0,
+    getScrollElement: () => scrollContainerRef.current,
+    estimateSize: () => virtualRowHeightPx,
+    overscan: 8,
+  });
+  const virtualRows = virtualEnabled ? virtualizer.getVirtualItems() : [];
+  const virtualPaddingTop = virtualRows[0]?.start ?? 0;
+  const virtualPaddingBottom = virtualEnabled
+    ? virtualizer.getTotalSize() - (virtualRows[virtualRows.length - 1]?.end ?? 0)
+    : 0;
+
   return (
     <div className="space-y-2">
-      <div className={cn('rounded-md border overflow-x-auto', cardRender && 'hidden lg:block')}>
+      <div
+        ref={virtualEnabled ? scrollContainerRef : undefined}
+        style={virtualEnabled ? { height: virtualHeightPx, overflow: 'auto' } : undefined}
+        className={cn('rounded-md border overflow-x-auto', cardRender && 'hidden lg:block')}
+      >
         <Table>
           <TableHeader className="sticky top-0 z-10 bg-muted/50">
             {table.getHeaderGroups().map((headerGroup) => (
@@ -246,8 +292,41 @@ export function DataTable<TData>({
                   )}
                 </TableCell>
               </TableRow>
+            ) : virtualEnabled ? (
+              <>
+                {virtualPaddingTop > 0 ? (
+                  <TableRow style={{ height: virtualPaddingTop }}>
+                    <TableCell colSpan={allColumns.length} className="p-0" />
+                  </TableRow>
+                ) : null}
+                {virtualRows.map((vRow) => {
+                  const row = rows[vRow.index]!;
+                  return (
+                    <TableRow
+                      key={row.id}
+                      data-state={row.getIsSelected() && 'selected'}
+                      className={cn(
+                        onRowClick && 'cursor-pointer',
+                        getRowClassName?.(row.original),
+                      )}
+                      onClick={() => onRowClick?.(row.original)}
+                    >
+                      {row.getVisibleCells().map((cell) => (
+                        <TableCell key={cell.id}>
+                          {flexRender(cell.column.columnDef.cell, cell.getContext())}
+                        </TableCell>
+                      ))}
+                    </TableRow>
+                  );
+                })}
+                {virtualPaddingBottom > 0 ? (
+                  <TableRow style={{ height: virtualPaddingBottom }}>
+                    <TableCell colSpan={allColumns.length} className="p-0" />
+                  </TableRow>
+                ) : null}
+              </>
             ) : (
-              table.getRowModel().rows.map((row) => (
+              rows.map((row) => (
                 <TableRow
                   key={row.id}
                   data-state={row.getIsSelected() && 'selected'}