Introduce sentence and word highlighting for HTML/TXT/MD documents, including new config options (`htmlHighlightEnabled`, `htmlWordHighlightEnabled`) and synced user preferences. Update the HTML viewer to support these highlights using custom spans, and add corresponding CSS styles. Extend document settings to allow toggling these features. Implement block-level locator sorting and display improvements. Add audiobook export support for HTML documents, including adapter and pipeline integration.
- Add `htmlHighlightEnabled` and `htmlWordHighlightEnabled` to config, user state, and Dexie storage
- Update HTML viewer to apply and manage highlights for sentences and words
- Add CSS for HTML highlight classes and scroll margin
- Extend document settings UI for HTML highlight toggles
- Support HTML audiobook export in modal and pipeline
- Improve block locator sorting and sidebar display for HTML
- Add HTML audiobook adapter and block parsing utilities
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