fix(koreader): keep the configured file name on downloads

Plugins from 1.7.0 request a delivery variant, and every such download had
" - bo-<id>-<variant>" added to its name. Only copies that would share a path are
renamed now: a narrated EPUB's text-only copy and later same-format files. A new
<a|b|c> chained default also stops the default pattern naming authorless
standalone books "Standalone/ - <title>". See docs/READ_ALOUD_DELIVERY.md.

Closes #1893

client/src/features/settings/file-naming/lib/pattern-highlight.ts

@@ -7,6 +7,7 @@
  *   {token:modifier} a placeholder with a transform
  *   <...>            an optional group, dropped when a placeholder inside it is empty
  *   <a|b>            the same, with `b` used when `a` cannot be filled
+ *   <a|b|c>          `b` only when it fills, otherwise `c`
  *   /                a path separator
  */
 export type PatternPieceKind = 'literal' | 'token' | 'modifier' | 'optional' | 'fallback' | 'separator'