Rename to Avrundningsfel, add Öresutjämning row, round total

Summary now shows:
- Product rows
- Avrundningsfel (rounding error)
- Totalt (rounded to nearest integer)
- Öresutjämning (difference from transactions to rounded total)

src/main.rs

@@ -86,6 +86,7 @@ struct Summary {
     total_volume: String,
     grand_total: String,
     products: Vec<ProductSummary>,
+    avrundningsfel: String,
     oresutjamning: String,
 }
 
@@ -192,12 +193,15 @@ impl PreparedCustomer {
             .map(|p| p.amount.parse::<f64>().unwrap())
             .sum();
 
-        let oresutjamning = total_from_transactions - grand_total;
+        let rounded_grand_total = grand_total.round();
+        let avrundningsfel = grand_total - rounded_grand_total;
+        let oresutjamning = total_from_transactions - rounded_grand_total;
 
         let summary = Summary {
             total_volume: fmt(total_volume),
-            grand_total: fmt(grand_total),
+            grand_total: fmt(rounded_grand_total),
             products,
+            avrundningsfel: fmt(avrundningsfel),
             oresutjamning: fmt(oresutjamning),
         };