fix(chess): finished games transition from active to completed

Wire onGameEndDismiss callback so the "Continue" button on game end
overlay moves the game to the completed list. Add auto-detection of
finished games during polling refresh so games transition even if the
user navigates away without clicking the button. Discovered games that
are already finished now go straight to completed instead of appearing
in the active list.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/chess/DesktopChessViewModelNew.kt

@@ -162,6 +162,8 @@ class DesktopChessViewModelNew(
 
     fun claimAbandonmentVictory(gameId: String) = logic.claimAbandonmentVictory(gameId)
 
+    fun dismissGame(gameId: String) = logic.dismissGame(gameId)
+
     // ============================================
     // Spectator operations
     // ============================================