feat: add complete eBook library and reader

[capex122]

Summary

• add the Harbor eBook library, source-plugin API, metadata discovery, filtering, collections, shelves, bookmarks, adaptations, and cross-source deduplication
• add the Harbor/Sard-inspired reader with navigation, line tracking, annotations, search, bookmarks, narration, DeepSeek translation, and persistent reading progress
• add local EPUB support plus EPUB/PDF offline exports integrated with Harbor downloads and download settings
• support HTML and binary gRPC eBook sources while preserving source volumes and chapters
• improve eBook covers, details, recommendations, context actions, source selection, and language-aware metadata

Validation

• TypeScript build passes: tsc -b --pretty false
• 36 focused eBook tests pass
• working tree was clean when the branch was pushed

Branches

• base: beta-branch
• head: eBook