feat: add money system with income, afford check, spend

src/engine/moneySystem.js

@@ -0,0 +1,20 @@
+// ==================== 银两系统 ====================
+
+export function applyDailyIncome(state, amount) {
+  state.money += amount;
+  if (state.money < 0) {
+    state.money = 0;
+  }
+}
+
+export function canAfford(state, cost) {
+  return state.money >= cost;
+}
+
+export function spendMoney(state, amount) {
+  if (!canAfford(state, amount)) {
+    return false;
+  }
+  state.money -= amount;
+  return true;
+}