When autoNext is enabled, audio automatically advances to the next chapter
when a track ends — navigating the page and starting the new chapter's audio.
- audioStore: add autoNext (toggle flag), nextChapter (written by AudioPlayer),
and autoStartPending (set before goto, cleared after auto-start fires)
- layout: update onended to call goto() and set autoStartPending when autoNext
is on and nextChapter is available; add auto-next toggle button to mini-player
bar (double-chevron icon, amber when active)
- AudioPlayer: accept nextChapter prop; write it to audioStore via $effect;
auto-start playback when autoStartPending is set on component mount; show
inline 'Auto' toggle button in ready controls when a next chapter exists
- Chapter page: pass data.next as nextChapter prop to AudioPlayer
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