v0.2.1

Fixed

• A remote book the source cannot serve says so. When the source no
  longer has the file behind an on-demand book, opening it now explains that
  in the reader instead of a bare HTTP status, and — when another copy of
  the same title on the shelf still opens — offers that copy right there.
  The book is remembered as unavailable, flagged in the Files list before the
  next tap, and cleared the moment a download succeeds again.
• Dead duplicates are pruned. When a re-imported copy of a book arrives
  next to an older record the source can no longer serve, the catalog sync
  removes the dead one (matched by title within the library, across
  shelves), and each sync also sweeps entries an open attempt already
  proved dead whose twin still serves. Needs a source that can probe a file
  cheaply; sources without that keep both copies.