perf(replies): progressive render, 500 replies/page, content-visibility

Set repliesPerPage to 500 to avoid Virtuoso pagination sort drift. Add useProgressiveRender with startTransition batching and CSS content-visibility for long reply lists.

src/lib/constants.ts

@@ -0,0 +1,2 @@
+/** Max replies loaded per page from useReplies. Threads with > this use Virtuoso. */
+export const REPLIES_PER_PAGE = 500;