Squash of feat/koreader-widget-board. KOReader plugin 2.0.0 gets the widget-board dashboard, redesigned
book details, menus, lists and dialogs, Decor page and reading companions. Highlights, notes and bookmarks
now stay in step across web, Kobo, KOReader and the phones (defects D-00 to D-20, migration 0121), and
reading progress and narration hand off cleanly between devices.
Introduce a sync coordinator to serialize and prioritize KOReader sync jobs (such as progress sync, highlight sync, library sweep, and update check). Coalesce pending jobs by family to prevent race conditions and redundant network requests. Use a dedicated scheduler for open annotations to handle highlight sync and matching asynchronously on book open. Add a diagnostics menu in the settings to track detailed sync status and provide clear error messages for debugging.