feat: preprocess offline PDF pages for Divina

[everpcpc]

Summary

• Preprocess downloaded PDF books for Divina by rendering pages into offline page image files (page-*.png) before reader load.
• Persist generated PDF pages and TOC into local metadata, and use a preparation stamp file to skip repeated preprocessing.
• Add OfflineManager helpers for writing offline page images, reading/writing PDF preparation stamp, and refreshing downloaded size.
• Skip Divina manifest refresh when the current book is already downloaded offline.
• Keep cache stats behavior intact and refresh downloaded size only when new offline PDF pages are rendered.

Testing

• make format
• make build