feat(library): rating line, manga finish-flip notify, game links, dedupe

- render **Rating:** line in manga/book/game/comic body (after read/play
  status, gated on rating != '0' + stars non-empty), matching watchlist's
  render.ts convention
- manga finish-flip now notifies user (final chapters out), matching
  chapter-flip's existing notify path
- game renderGame emits ## Links (Steam by appid, RAWG by url) — fixes
  latent owned-heading strip where user-added Links sections silently
  vanished since games never re-rendered the heading
- move duplicated deriveReadStatus/deriveRating/parseNumOrNull into
  library/convert.ts, import across specs; zero behavior change

tests/fixtures/canonical-book.md

@@ -22,6 +22,7 @@ tags: [books, book]
 **Book** · 1949 · 328 p.
 
 **Read Status:** Read
+**Rating:** ⭐️⭐️⭐️⭐️⭐️ (5/5)
 
 **Authors:** George Orwell