Fix apostrophe encoding in chapter names (issue #181) (#182)

The clean_pgn() function unescaped > and " HTML entities but
was missing ' (apostrophe), causing chapter names like "King's
English" to display as "King's English" in the sidebar.

https://claude.ai/code/session_01RGwiXshb2J84P5QQxDzGjr

Author: IngridMorstrad

lib/listudy_web/templates/study/show.html.eex

@@ -110,6 +110,7 @@
     pgn = pgn.replace(/>/g, '>');
     pgn = pgn.replace(/&lt;/g, '<');
     pgn = pgn.replace(/&quot;/g, '"');
+    pgn = pgn.replace(/&#39;/g, "'");
     // U+FEFF ZERO WIDTH NO-BREAK SPACE
     // No idea why they are in some pgns but they break the parser 
     // https://github.com/ArneVogel/listudy/issues/35