fix: improve VAT rounding and add per-line VAT display support

Fix calculateVat/calculateTotal to use proper monetary rounding. Add
getVatSummaryFromItems helper for deriving VAT labels from mixed-rate items.
Update invoice preview, review, and detail pages to show per-rate VAT
breakdown instead of a single aggregated rate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/invoices/pdf-template.tsx

@@ -440,7 +440,7 @@ export function InvoicePDF({ invoice, customer, items, company, originalInvoiceN
                 ))
             ) : (
               <View style={styles.totalRow}>
-                <Text style={styles.totalLabel}>Moms ({invoice.vat_rate ?? 25}%):</Text>
+                <Text style={styles.totalLabel}>Moms ({invoice.vat_rate ?? (vatByRate.size === 1 ? vatByRate.keys().next().value : 25)}%):</Text>
                 <Text style={styles.totalValue}>{formatCurrency(invoice.vat_amount, invoice.currency)}</Text>
               </View>
             )}

lib/invoices/vat-rules.ts

@@ -118,14 +118,14 @@ export function getVatRules(
  * Calculate VAT amount
  */
 export function calculateVat(subtotal: number, vatRate: number): number {
-  return subtotal * (vatRate / 100)
+  return Math.round(subtotal * vatRate) / 100
 }
 
 /**
  * Calculate total including VAT
  */
 export function calculateTotal(subtotal: number, vatRate: number): number {
-  return subtotal + calculateVat(subtotal, vatRate)
+  return Math.round((subtotal + calculateVat(subtotal, vatRate)) * 100) / 100
 }
 
 /**
@@ -153,6 +153,36 @@ export function getVatTreatmentLabel(treatment: VatTreatment): string {
   return labels[treatment]
 }
 
+/**
+ * Derive a display-friendly VAT summary from invoice line items.
+ *
+ * - If all items share a single rate → returns that rate's label and treatment
+ * - If items have mixed rates → returns "Blandade momssatser" with null rate/treatment
+ */
+export function getVatSummaryFromItems(
+  items: { vat_rate?: number | null }[]
+): { label: string; treatment: VatTreatment | null; rate: number | null; isMixed: boolean } {
+  const rates = new Set(items.map((item) => item.vat_rate ?? 25))
+
+  if (rates.size === 1) {
+    const rate = rates.values().next().value!
+    const treatment = getVatTreatmentForRate(rate)
+    return {
+      label: getVatTreatmentLabel(treatment),
+      treatment,
+      rate,
+      isMixed: false,
+    }
+  }
+
+  return {
+    label: 'Blandade momssatser',
+    treatment: null,
+    rate: null,
+    isMixed: true,
+  }
+}
+
 /**
  * Get moms ruta description
  */