fix: overhaul Ludo multiplayer sync — remove dual-poller, add state diffing + proper resets

Eliminates the conflict between matchLive.start() and the game's own
startLudoPolling() which caused race conditions. Consolidates to a single
poller with: proper turn_count diffing, remote dice display via
showRemoteDice(), explicit game.rolled reset on turn transitions, stale
dice display fix, diceAnimating stuck-flag safety valve, double-poll
prevention in handleNonPlayerTurn, heartbeat timer cleanup, and an
unmountGame export for proper teardown.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>