feat(chess): exit spectator mode with Leave Game button

Adds ability to leave a spectated game on both Android and Desktop:
- Added stopSpectating() to ChessLobbyLogic (removes from state + stops polling)
- Both ViewModels now delegate to logic.stopSpectating() instead of bypassing it
- LiveChessGameScreen accepts onLeaveSpectating callback shown in spectator info area
- Android: Leave Game button pops back stack and stops spectating
- Desktop: Leave Game button clears selectedGameId (returns to lobby) and stops spectating

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

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chess/ChessGameScreen.kt

@@ -350,6 +350,15 @@ fun ChessGameScreen(
                     onResign = { chessViewModel.resign(gameId) },
                     isSpectatorOverride = isSpectating,
                     onGameEndDismiss = { chessViewModel.dismissGame(gameId) },
+                    onLeaveSpectating =
+                        if (isSpectating) {
+                            {
+                                chessViewModel.stopSpectating(gameId)
+                                nav.popBack()
+                            }
+                        } else {
+                            null
+                        },
                     whiteName = whiteDisplayName,
                     whiteHex = whitePubkey,
                     whiteAvatarUrl = whiteAvatarUrl,

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/chess/ChessViewModelNew.kt

@@ -176,7 +176,7 @@ class ChessViewModelNew(
 
     fun loadGameAsSpectator(gameId: String) = logic.loadGameAsSpectator(gameId)
 
-    fun stopSpectating(gameId: String) = logic.state.removeSpectatingGame(gameId)
+    fun stopSpectating(gameId: String) = logic.stopSpectating(gameId)
 
     // ============================================
     // Utility