fix: Correct PnL calculation by using betAmount in net profit computation

- Add betAmount field to BlackjackState to track the base bet
- Fix finishPlayerTurns: multiply hand.bet by state.betAmount for actual money bets
- Fix GameServer: roundPayouts are already gross payouts (not multipliers)
- Update cumulative PnL calculation to correctly subtract actual bet amount
- Add betAmount support to riggedState test helper

shared/games/blackjack/blackjack.plugin.test.ts

@@ -34,6 +34,7 @@ function riggedState(overrides: {
     activePlayerIndex?: number;
     phase?: "betting" | "player_turns" | "resolved";
     roundNumber?: number;
+    betAmount?: number;
 }): BlackjackState {
     return {
         deck: overrides.deck ?? [makeCard("5"), makeCard("6"), makeCard("7"), makeCard("8"), makeCard("9"), makeCard("10")],
@@ -43,6 +44,7 @@ function riggedState(overrides: {
         activePlayerIndex: overrides.activePlayerIndex ?? 0,
         phase: overrides.phase ?? "player_turns",
         roundNumber: overrides.roundNumber ?? 1,
+        betAmount: overrides.betAmount ?? 0,
     };
 }