fix: 修复当前 USDT 余额计算，确保在余额不存在时返回 0

src/components/subscribe-rwa/index.vue

@@ -13,8 +13,9 @@ const emit = defineEmits<{
 const { t } = useI18n();
 
 const walletStore = useWalletStore();
+await walletStore.syncFundingBalances();
 const { fundingBalances } = storeToRefs(walletStore);
-const currentUSDTBalance = computed(() => fundingBalances.value[0].available);
+const currentUSDTBalance = computed(() => fundingBalances.value[0]?.available || 0);
 
 const num = ref<number | null>(null);