Cap product combo qty by real stock instead of a fixed 3 per line

Live E2E testing against the real Sổ Bán Lẻ catalog (64 usable
products, avg price ~24k) showed the fixed maxQtyPerLine=3 capped the
achievable order total at ~4.86M regardless of retry count - a
structural ceiling, not bad luck. 7/10 test amounts above ~4.2M failed
with NO_PRODUCT_COMBO_MATCH every time.

Default per-line qty is now bounded only by the product's actual stock
(previously min(3, qty), now just qty), raising the real ceiling to
~2B on the current catalog. maxQtyPerLine is still available as an
explicit override for callers that want a cap.

Verified against the real catalog: all 7 previously-failing amounts
(4.2M-19.5M) now succeed, real order totals landing within the
50,000 tolerance band every time.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017KPzWwuTEeX2vXGXvyGn4q

app/libs/productComboPicker.js

@@ -2,7 +2,6 @@
 
 var DEFAULT_TOLERANCE = 50000;
 var DEFAULT_MAX_ATTEMPTS = 20;
-var DEFAULT_MAX_QTY_PER_LINE = 3;
 
 function shuffle(list, random) {
   var arr = list.slice();
@@ -28,7 +27,7 @@ function pickProductCombo(products, targetAmount, options) {
   var tolerance = typeof options.tolerance === "number" ? options.tolerance : DEFAULT_TOLERANCE;
   var maxAttempts = options.maxAttempts || DEFAULT_MAX_ATTEMPTS;
   var random = options.random || Math.random;
-  var maxQtyPerLine = options.maxQtyPerLine || DEFAULT_MAX_QTY_PER_LINE;
+  var maxQtyPerLine = typeof options.maxQtyPerLine === "number" ? options.maxQtyPerLine : null;
 
   var candidates = products.filter(function (p) {
     return p.qty > 0 && p.price > 0;
@@ -46,7 +45,7 @@ function pickProductCombo(products, targetAmount, options) {
 
     for (var i = 0; i < shuffled.length; i++) {
       var product = shuffled[i];
-      var maxQty = Math.min(maxQtyPerLine, product.qty);
+      var maxQty = maxQtyPerLine === null ? product.qty : Math.min(maxQtyPerLine, product.qty);
       var qty = 1 + Math.floor(random() * maxQty);
       var lineTotal = computeLineTotal(product, qty);