9f3e895fa8
When autoNext is on, kick off prefetchNext() as soon as the current chapter begins playing (all three success paths in startPlayback). This gives the full chapter duration as lead time for Kokoro TTS instead of only the last 10%, making auto-next transitions seamless. The existing 90%-mark $effect is kept as a fallback for cases where autoNext is toggled on mid-playback after startPlayback has returned.