fix: guard against hasMore=true with nextCursor=null in V4 path

When V4 returns hasMore=true but nextCursor=null, the next load-more
call would pass cursor=null, triggering the replace branch instead of
append. Treat this edge case as 'done' to prevent silent list reset.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sethconvex

src/routes/skills/-useSkillsBrowseModel.ts

@@ -83,8 +83,9 @@ export function useSkillsBrowseModel({
           })
           if (generation !== fetchGeneration.current) return
           setListResults((prev) => (cursor ? [...prev, ...result.page] : result.page))
-          setListCursor(result.hasMore ? (result.nextCursor ?? null) : null)
-          setListStatus(result.hasMore ? 'idle' : 'done')
+          const canAdvance = result.hasMore && result.nextCursor != null
+          setListCursor(canAdvance ? result.nextCursor : null)
+          setListStatus(canAdvance ? 'idle' : 'done')
         } else {
           const result = await convexHttp.query(api.skills.listPublicPageV3, {
             paginationOpts: { cursor, numItems: pageSize },