perf: add export loading state and virtual scrolling for large lists

Export modal now shows a loading indicator and doesn't freeze the UI.
Large list modals use virtual scrolling for smooth performance.

components/dashboard/VirtualList.tsx

@@ -0,0 +1,53 @@
+'use client'
+
+import { useRef } from 'react'
+import { useVirtualizer } from '@tanstack/react-virtual'
+
+interface VirtualListProps<T> {
+  items: T[]
+  estimateSize: number
+  className?: string
+  renderItem: (item: T, index: number) => React.ReactNode
+}
+
+export default function VirtualList<T>({ items, estimateSize, className, renderItem }: VirtualListProps<T>) {
+  const parentRef = useRef<HTMLDivElement>(null)
+
+  const virtualizer = useVirtualizer({
+    count: items.length,
+    getScrollElement: () => parentRef.current,
+    estimateSize: () => estimateSize,
+    overscan: 10,
+  })
+
+  // For small lists (< 50 items), render directly without virtualization
+  if (items.length < 50) {
+    return (
+      <div className={className}>
+        {items.map((item, index) => renderItem(item, index))}
+      </div>
+    )
+  }
+
+  return (
+    <div ref={parentRef} className={className} style={{ overflow: 'auto' }}>
+      <div style={{ height: `${virtualizer.getTotalSize()}px`, width: '100%', position: 'relative' }}>
+        {virtualizer.getVirtualItems().map((virtualRow) => (
+          <div
+            key={virtualRow.key}
+            style={{
+              position: 'absolute',
+              top: 0,
+              left: 0,
+              width: '100%',
+              height: `${virtualRow.size}px`,
+              transform: `translateY(${virtualRow.start}px)`,
+            }}
+          >
+            {renderItem(items[virtualRow.index], virtualRow.index)}
+          </div>
+        ))}
+      </div>
+    </div>
+  )
+}