perf: defer board reply fetching and remove nested mobile backlink fetch

Split useReplies in post-desktop.tsx and post-mobile.tsx into preview/full modes so board cards avoid full REPLIES_PER_PAGE fetches by default. Added replies-preview-utils.ts and switched mobile backlinks to use precomputed directRepliesByParentCid map instead of per-reply useReplies.

src/lib/utils/replies-preview-utils.ts

@@ -0,0 +1,48 @@
+import { BOARD_REPLIES_PREVIEW_VISIBLE_COUNT } from '../constants';
+
+export interface CommentLike {
+  cid?: string | null;
+}
+
+/**
+ * From replies sorted by 'new' (newest first), returns the latest N in chronological
+ * display order (oldest of those first). Handles fewer-than-N replies.
+ */
+export function getPreviewDisplayReplies<T extends CommentLike>(replies: T[], visibleCount: number = BOARD_REPLIES_PREVIEW_VISIBLE_COUNT): T[] {
+  const slice = replies.slice(0, visibleCount);
+  return [...slice].reverse();
+}
+
+export interface ComputeOmittedParams {
+  totalReplyCount: number;
+  visibleCount: number;
+  pinned?: boolean;
+}
+
+/**
+ * Computes omitted reply count for board view. For pinned threads, collapsed view
+ * shows 0 replies, so omitted = total. For non-pinned, omitted = total - visible.
+ * Result is always clamped at zero.
+ */
+export function computeOmittedCount({ totalReplyCount, visibleCount, pinned = false }: ComputeOmittedParams): number {
+  if (pinned) {
+    return Math.max(0, totalReplyCount);
+  }
+  return Math.max(0, totalReplyCount - visibleCount);
+}
+
+export interface GetTotalReplyCountParams {
+  replyCount: number | undefined;
+  fullLoadedCount: number;
+  previewLoadedCount: number;
+}
+
+/**
+ * Returns total reply count: post.replyCount when defined, else max of loaded counts.
+ */
+export function getTotalReplyCount({ replyCount, fullLoadedCount, previewLoadedCount }: GetTotalReplyCountParams): number {
+  if (typeof replyCount === 'number' && replyCount >= 0) {
+    return replyCount;
+  }
+  return Math.max(fullLoadedCount, previewLoadedCount);
+}