fix: viewer hasNextPage predicate off-by-boundary (Playlist/Artist)

[KazeKaze93]

Summary

• Playlist/Artist openViewer now pass the react-query hasNextPage flag instead of a loaded-count vs page-size / totalCount predicate. A full last page (50/50) no longer freezes the viewer next-arrow while more data exists.
• Browse already passed the query flag; unchanged. Background gallery fetch without appendQueueIds (P3) is out of scope.
• Source-scan regression test locks all five galleries to hasNextPage: hasNextPage.

Test plan

• Playlist with >50 posts: open viewer on last card of the first loaded page (exactly 50), press next — page 2 loads, navigation continues
• Artist with >50 posts: same first-page boundary; next continues (and totalGlobalCount still stops when loaded count is exhausted)
• Playlist/Artist last page not a multiple of 50 — no regression
• Exhausted data (short last page, query hasNextPage === false) — next-arrow stops
• npm run typecheck / lint / test — already green on the branch (46 files / 312 tests)