feat(audiobook): add chapter-based export with UI and API
Introduce end-to-end chapterized audiobook generation with persistent
storage, resumable workflows, and MP3/M4B support.
API:
- add /api/audio/convert/chapter (GET/DELETE) for per-chapter ops
- add /api/audio/convert/chapters (GET/DELETE) for listing/reset
- enhance /api/audio/convert:
- accept mp3|m4b, stream combined file, cache complete output
- robust chapter indexing, docstore persistence, list concat
- AbortSignal-aware ffmpeg/ffprobe, 499 on cancel
UI/UX:
- add AudiobookExportModal with progress, resume, regenerate, download
- add ProgressCard, enhance ProgressPopup (click-to-focus, richer info)
- add Header, ZoomControl; move TTSPlayer to sticky bottom bar
- redesign EPUB/HTML/PDF pages for full-height layout and controls
- add HomeContent; compact uploader variant; document list polish
TTS/Contexts:
- EPUB/PDF contexts now generate per-chapter to disk, return bookId
- support chapter regeneration; progress/cancel propagation
- pass provider/model/instructions; standardize MP3 TTS output
- HTML context updates for model/instructions
Styling:
- globals: overlay-dim, scrollbar styles, prism gradient utilities
- theme vars: secondary-accent, prism-gradient; tailwind color addition
Misc:
- fix PDF scale calc to use container height
- EPUB theme reader area fills height
- time estimation update cadence stab
- audio util passes format to combine endpoint
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