feat: CNB exchange rates, multi-currency KPI stats, invoice PDF VAT in CZK

- ČNB exchange rate service with date-specific rates and caching
- Invoice/received invoice stats convert foreign currencies to CZK
- Dashboard revenue converts all currencies to CZK
- Invoice PDF: VAT recap table always in CZK with CNB rate footer
- Inline styles replaced with utility classes (step 4 cleanup)
- Spinner animation exempt from prefers-reduced-motion

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/invoices.service.ts

@@ -1,4 +1,5 @@
 import prisma from "../config/database";
+import { toCzk } from "./exchange-rates";
 
 // Status transition rules matching PHP
 const VALID_TRANSITIONS: Record<string, string[]> = {
@@ -186,10 +187,11 @@ export async function getInvoiceStats(queryMonth?: number, queryYear?: number) {
       }));
   };
 
-  const sumCzk = (invoices: typeof allInvoices) => {
+  const sumCzk = async (invoices: typeof allInvoices) => {
     let total = 0;
     for (const inv of invoices) {
-      total += invoiceTotalWithVat(inv); // Simplified: no real FX conversion
+      const amount = invoiceTotalWithVat(inv);
+      total += await toCzk(amount, inv.currency || "CZK");
     }
     return Math.round(total * 100) / 100;
   };
@@ -224,18 +226,24 @@ export async function getInvoiceStats(queryMonth?: number, queryYear?: number) {
   let vatCzk = 0;
   for (const [, v] of Object.entries(vatMap)) vatCzk += v;
 
+  // VAT also needs conversion
+  let vatCzkConverted = 0;
+  for (const [cur, amount] of Object.entries(vatMap)) {
+    vatCzkConverted += await toCzk(amount, cur);
+  }
+
   return {
     paid_month: aggregateByCurrency(paidInvoices),
-    paid_month_czk: sumCzk(paidInvoices),
+    paid_month_czk: await sumCzk(paidInvoices),
     paid_month_count: paidInvoices.length,
     awaiting: aggregateByCurrency(awaitingInvoices),
-    awaiting_czk: sumCzk(awaitingInvoices),
+    awaiting_czk: await sumCzk(awaitingInvoices),
     awaiting_count: awaitingInvoices.length,
     overdue: aggregateByCurrency(overdueInvoices),
-    overdue_czk: sumCzk(overdueInvoices),
+    overdue_czk: await sumCzk(overdueInvoices),
     overdue_count: overdueInvoices.length,
     vat_month: vatAmounts,
-    vat_month_czk: Math.round(vatCzk * 100) / 100,
+    vat_month_czk: Math.round(vatCzkConverted * 100) / 100,
     month,
     year,
   };