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>

app/(dashboard)/invoices/[id]/page.tsx

@@ -512,10 +512,33 @@ export default function InvoiceDetailPage({ params }: { params: Promise<{ id: st
                     <span className="text-muted-foreground">Delsumma</span>
                     <span>{formatCurrency(invoice.subtotal, invoice.currency)}</span>
                   </div>
-                  <div className="flex justify-between">
-                    <span className="text-muted-foreground">Moms ({invoice.vat_rate}%)</span>
-                    <span>{formatCurrency(invoice.vat_amount, invoice.currency)}</span>
-                  </div>
+                  {(() => {
+                    const vatByRate = new Map<number, number>()
+                    for (const item of invoice.items) {
+                      const rate = item.vat_rate ?? 25
+                      const lineVat = Math.round(item.line_total * (rate / 100) * 100) / 100
+                      vatByRate.set(rate, (vatByRate.get(rate) || 0) + lineVat)
+                    }
+                    const entries = Array.from(vatByRate.entries())
+                      .filter(([, vat]) => vat > 0)
+                      .sort(([a], [b]) => b - a)
+
+                    if (entries.length === 0) {
+                      return (
+                        <div className="flex justify-between">
+                          <span className="text-muted-foreground">Moms</span>
+                          <span>{formatCurrency(0, invoice.currency)}</span>
+                        </div>
+                      )
+                    }
+
+                    return entries.map(([rate, vat]) => (
+                      <div key={rate} className="flex justify-between">
+                        <span className="text-muted-foreground">Moms {rate}%</span>
+                        <span>{formatCurrency(vat, invoice.currency)}</span>
+                      </div>
+                    ))
+                  })()}
                   <Separator />
                   <div className="flex justify-between font-bold text-lg">
                     <span>Totalt</span>

app/(dashboard)/invoices/new/page.tsx

@@ -16,7 +16,7 @@ import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from '@
 import { Separator } from '@/components/ui/separator'
 import { useToast } from '@/components/ui/use-toast'
 import { formatCurrency } from '@/lib/utils'
-import { getVatRules, getVatTreatmentLabel, getAvailableVatRates } from '@/lib/invoices/vat-rules'
+import { getVatRules, getAvailableVatRates, getVatSummaryFromItems } from '@/lib/invoices/vat-rules'
 import { Dialog, DialogContent, DialogHeader, DialogTitle, DialogDescription, DialogFooter } from '@/components/ui/dialog'
 import { Loader2, Plus, Trash2, ArrowLeft, Send, Eye } from 'lucide-react'
 import { ConfirmationDialog } from '@/components/ui/confirmation-dialog'
@@ -369,7 +369,7 @@ export default function NewInvoicePage() {
                 {selectedCustomer && vatRules && (
                   <div className="mt-4 p-3 bg-muted rounded-lg">
                     <p className="text-sm">
-                      <strong>Momsbehandling:</strong> {getVatTreatmentLabel(vatRules.treatment)}
+                      <strong>Momsbehandling:</strong> {getVatSummaryFromItems(watchItems).label}
                     </p>
                     {vatRules.reverseChargeText && (
                       <p className="text-xs text-muted-foreground mt-1">
@@ -664,10 +664,8 @@ export default function NewInvoicePage() {
               vat_rate: item.vat_rate ?? (vatRules?.rate || 25),
             }))}
             subtotal={subtotal}
-            vatRate={vatRules.rate}
             vatAmount={vatAmount}
             total={total}
-            vatTreatment={vatRules.treatment}
             yourReference={pendingData?.your_reference}
             ourReference={pendingData?.our_reference}
             notes={pendingData?.notes}

app/api/invoices/preview-pdf/route.ts

@@ -55,25 +55,33 @@ export async function POST(request: Request) {
   const docType: InvoiceDocumentType = document_type || 'invoice'
   const isDeliveryNote = docType === 'delivery_note'
 
-  // Build items with line totals
-  const invoiceItems: InvoiceItem[] = items.map((item: { description: string; quantity: number; unit: string; unit_price: number; vat_rate?: number }, index: number) => ({
-    id: `preview-${index}`,
-    invoice_id: 'preview',
-    sort_order: index,
-    description: item.description,
-    quantity: item.quantity,
-    unit: item.unit,
-    unit_price: item.unit_price,
-    line_total: Math.round(item.quantity * item.unit_price * 100) / 100,
-    vat_rate: item.vat_rate ?? vatRules.rate,
-    vat_amount: 0,
-    created_at: new Date().toISOString(),
-  }))
+  // Build items with line totals and per-item VAT
+  const invoiceItems: InvoiceItem[] = items.map((item: { description: string; quantity: number; unit: string; unit_price: number; vat_rate?: number }, index: number) => {
+    const lineTotal = Math.round(item.quantity * item.unit_price * 100) / 100
+    const rate = item.vat_rate ?? vatRules.rate
+    return {
+      id: `preview-${index}`,
+      invoice_id: 'preview',
+      sort_order: index,
+      description: item.description,
+      quantity: item.quantity,
+      unit: item.unit,
+      unit_price: item.unit_price,
+      line_total: lineTotal,
+      vat_rate: rate,
+      vat_amount: isDeliveryNote ? 0 : Math.round(lineTotal * (rate / 100) * 100) / 100,
+      created_at: new Date().toISOString(),
+    }
+  })
 
   const subtotal = invoiceItems.reduce((sum, item) => sum + item.line_total, 0)
-  const vatAmount = isDeliveryNote ? 0 : Math.round(subtotal * (vatRules.rate / 100) * 100) / 100
+  const vatAmount = isDeliveryNote ? 0 : invoiceItems.reduce((sum, item) => sum + item.vat_amount, 0)
   const total = isDeliveryNote ? 0 : subtotal + vatAmount
 
+  // Derive vat_rate from items: single rate → that rate, mixed → null
+  const itemRates = new Set(invoiceItems.map((item) => item.vat_rate))
+  const effectiveVatRate = isDeliveryNote ? 0 : (itemRates.size === 1 ? itemRates.values().next().value! : null)
+
   // Construct a temporary Invoice-like object
   const previewInvoice = {
     id: 'preview',
@@ -93,7 +101,7 @@ export async function POST(request: Request) {
     total,
     total_sek: null,
     vat_treatment: vatRules.treatment,
-    vat_rate: isDeliveryNote ? 0 : vatRules.rate,
+    vat_rate: effectiveVatRate,
     moms_ruta: vatRules.momsRuta,
     your_reference: your_reference || null,
     our_reference: our_reference || null,