Add an optional cover fetcher to the latest-chapter poller and wire the shared browser into it. Each due Kagane series now checks persistent storage, fetches a missing cover once, and keeps chapter polling and cooldown bookkeeping independent when cover work fails.\n\nCentralize Kagane image-id and content-type validation so prefetched bytes cannot poison the on-first-view fallback, and cover the missing, cached, failure, nil-fetcher, invalid-type, and non-Kagane paths with deterministic poll-cycle tests.\n\nRefs #45
sulthan
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