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/library-book.test.ts

@@ -149,6 +149,15 @@ describe('renderBook golden', () => {
 		expect(out).toContain('## My Notes\n\nreread every few years');
 	});
 
+	test('rating line present when rating set', () => {
+		expect(renderBook(RECORD, '')).toContain('**Rating:** ⭐️⭐️⭐️⭐️⭐️ (5/5)');
+	});
+
+	test('rating 0 -> Rating line absent', () => {
+		const r = { ...RECORD, rating: '0', ratingStars: '' };
+		expect(renderBook(r, '')).not.toContain('**Rating:**');
+	});
+
 	test('no goodreads url -> Links has only Open Library entry', () => {
 		const out = renderBook(RECORD, '');
 		expect(out).toContain('- [Open Library](https://openlibrary.org/works/OL1168083W)');