Ensure grand_total is consistent with product calculations

Calculate grand_total from products (volume * avg_price), matching the product row calculations.

src/main.rs

@@ -149,11 +149,6 @@ impl PreparedCustomer {
             })
             .collect();
 
-        let grand_total: f64 = cards
-            .iter()
-            .map(|c| c.total_amount.parse::<f64>().unwrap())
-            .sum();
-
         let mut product_totals: HashMap<String, (f64, f64)> = HashMap::new();
         for card in &cards {
             for tx in &card.transactions {
@@ -187,6 +182,11 @@ impl PreparedCustomer {
             .map(|p| p.volume.parse::<f64>().unwrap())
             .sum();
 
+        let grand_total: f64 = products
+            .iter()
+            .map(|p| p.amount.parse::<f64>().unwrap())
+            .sum();
+
         let summary = Summary {
             total_volume: fmt(total_volume),
             grand_total: fmt(grand_total),