Fix playlist playback repeating after 50 videos

[HankFreeman1]

For #767, this PR fixes playlist playback looping back to previously played videos after the initial batch.

The issue was in the playlist prefetch logic in PlaylistPlaybackPage. When the player approached the end of the loaded items and triggered a prefetch, the code used currentIndex (the current playback position) as the starting point for fetching new items, instead of count (the current playlist size). This meant:

1. Items already in the playlist were being re-fetched from the pager and appended as duplicates
2. The upper bound currentIndex + PLAYLIST_PREFETCH didn't extend far enough past the existing playlist, so new items beyond the initial batch were never loaded

The fix changes both the range start and upper bound to use count, so new items are always fetched starting from where the playlist currently ends rather than from the current playback position.

I tested locally and this does solve my issue!

[damontecres]

Good catch, thanks!