Add Option to Fallback to Series Cover when syncing Book Covers

[mjoetodd]

Summary

For users who wish to have their book covers match their series covers when no other cover option exists. Primary use case is Komga users who wish to see book/chapter covers on their home screens (i.e., Keep Reading, On Deck, Recently Added) that is more visually appealing than the first page of that book/chapter. Many users lack local artwork to leverage Komga's native cover capability for each book and sources used by Komf may have have limited book covers for Webtoon, Manhwa, and so on. This option augments that native series/book update capability in Komf to use the series cover in the event a book cover was not found.

Behavior

• Flag name: fallbackUseSeriesCoverForBook (default: false)
• Only when processing configuration is true for the following options: bookCovers, seriesCovers, and fallbackUseSeriesCoverForBook
• Komf will apply the Series Cover to the Book Cover Metadata, only when no Book Cover Metadata was returned by the provider.

Testing

• Feature was tested across several providers (single and aggregate) for three library types (Manga, Manhwa, and Webtoon)
• Feature was tested against a large Komga library (~140 series & +17k books)
• Was not tested in Kavita
• Could not directly test new web browser extensions (option unavailable).

I welcome any feedback.

Note: This update leveraged Chatgpt Codex to assist in the development and documentation. I'm unaware if this conflicts with the repo's development philosophy.