uploadBook only attached a cover.png when one was already cached under
readest_covers/<hash>.png from a prior cloud download. Books that
originated locally in KOReader were never downloaded, so the cover step
was silently skipped and they synced to Readest with no cover.
Add extractLocalCover, which renders the book's embedded cover via
coverbrowser's FileManagerBookInfo:getCoverImage(nil, file_path) and
writes it as PNG. uploadBook now falls back to it when no cached cover
exists, caching the result under covers_dir so the Library view reuses
it like a downloaded cover.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Huang Xin
a92fe0ce2e