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
Implement strict validation for EPUB TTS segment locators, requiring stable spine coordinates (`spineHref`, `spineIndex`, `charOffset`) and rejecting legacy CFI-only locators. Introduce `segmentKey` as a normalized identity derived from segment text, used for robust merging and deduplication of synthesized and persisted segments. Update database schemas, API routes, and manifest logic to support the new locator format and segmentKey. Add helpers for locator resolution and segmentKey construction, with comprehensive unit tests for identity, normalization, and manifest grouping.
BREAKING CHANGE: Persisted EPUB TTS segment locators must now include stable spine coordinates; legacy CFI-only locators are no longer accepted. Consumers must handle the new `segmentKey` field for segment identity and merging.
