fix: prevent billing process total from rounding to 0 for very small amounts (e.g. low-ratio channel-only models)

- Add formatBillingTotalDisplay: auto-scales from 2 to 6 decimal places when amount < 0.005
- Replace formatBillingDisplayPrice with formatBillingTotalDisplay in the billing total line
- Prevents '= \' display when actual cost is a tiny positive value (e.g. 0.00002 USD)

web/src/helpers/render.jsx

@@ -1503,6 +1503,19 @@ function formatBillingDisplayPrice(usdAmount, rate, digits = 2) {
   return parseFloat((usdAmount * rate).toFixed(digits));
 }
 
+/**
+ * 计费过程总额专用格式化：正常金额保持 2 位小数；极小金额（< 0.005）自动提升至 6 位小数，
+ * 避免因 toFixed(2) 归零导致「计费过程」显示为 0。
+ */
+function formatBillingTotalDisplay(usdAmount, rate) {
+  const raw = usdAmount * rate;
+  if (raw <= 0) return 0;
+  const fixed2 = Number(raw.toFixed(2));
+  if (fixed2 > 0) return fixed2;
+  const fixed6 = Number(raw.toFixed(6));
+  return fixed6 > 0 ? fixed6 : 0;
+}
+
 function buildBillingText(key, vars) {
   return i18next.t(key, vars);
 }
@@ -2529,7 +2542,7 @@ export function renderModelPrice(
           outputDesc,
           extraServices,
           symbol,
-          total: formatBillingDisplayPrice(price, rate),
+          total: formatBillingTotalDisplay(price, rate),
         },
       ),
     ];