- modal.js: dismiss existing modal before showing new one (prevents stacking)
- scene.js: call unmount on current scene during replace() and switchWorld()
- chat.js: remove || true from scroll condition (only auto-scroll on new msgs)
- shop/browse.js: check gems affordability alongside coins
- ludo/game.js: fix undefined isLoser — use isLastPlace
- backgammon/rules.js: fix VARIANTS.standard crash — use VARIANTS.sheshbesh
- backgammon/game.js: sanitize opponentName and avatar URL (XSS prevention)

Fixes WTF #22, #25, #26, #59, #68, #71, #113
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>