feat: add SPA, serve at /app/, update Dockerfile and README

- React + TanStack Router + shadcn/ui SPA under spa/
- serve spa/dist at /app/ with index.html fallback for client routing
- Dockerfile: node build stage for SPA, copy dist into runtime image
- README: document SPA, CORS_ORIGINS env var, architecture entry
- vite base set to /app/, manifest.json paths fixed

spa/src/components/virtual-list.tsx

@@ -0,0 +1,70 @@
+import { useEffect, useRef } from "react"
+import { useWindowVirtualizer } from "@tanstack/react-virtual"
+import { Spinner } from "@/components/ui/spinner"
+
+type VirtualListProps<T> = {
+  items: T[]
+  estimateSize: number
+  renderItem: (item: T, index: number) => React.ReactNode
+  hasMore?: boolean
+  isFetching?: boolean
+  onLoadMore?: () => void
+  overscan?: number
+}
+
+export function VirtualList<T>({
+  items,
+  estimateSize,
+  renderItem,
+  hasMore = false,
+  isFetching = false,
+  onLoadMore,
+  overscan = 5,
+}: VirtualListProps<T>) {
+  const listRef = useRef<HTMLDivElement>(null)
+
+  const virtualizer = useWindowVirtualizer({
+    count: items.length,
+    estimateSize: () => estimateSize,
+    overscan,
+    scrollMargin: listRef.current?.offsetTop ?? 0,
+  })
+
+  const virtualItems = virtualizer.getVirtualItems()
+  const lastItem = virtualItems.at(-1)
+
+  useEffect(() => {
+    if (!lastItem || !hasMore || isFetching || !onLoadMore) return
+    if (lastItem.index >= items.length - 5) {
+      onLoadMore()
+    }
+  }, [lastItem?.index, items.length, hasMore, isFetching, onLoadMore])
+
+  return (
+    <div ref={listRef}>
+      <div
+        className="relative w-full"
+        style={{ height: virtualizer.getTotalSize() }}
+      >
+        {virtualItems.map((virtualRow) => (
+          <div
+            key={virtualRow.key}
+            data-index={virtualRow.index}
+            ref={virtualizer.measureElement}
+            className="absolute left-0 top-0 w-full"
+            style={{ transform: `translateY(${virtualRow.start - (virtualizer.options.scrollMargin ?? 0)}px)` }}
+          >
+            <div className="pb-2">
+              {renderItem(items[virtualRow.index]!, virtualRow.index)}
+            </div>
+          </div>
+        ))}
+      </div>
+      {isFetching && (
+        <div className="flex justify-center py-4">
+          <Spinner className="size-5" />
+        </div>
+      )}
+    </div>
+  )
+}