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/admin/pages/Invoices.tsx

@@ -56,7 +56,7 @@ function formatCzkWithDetail(
   if (!Array.isArray(amounts) || amounts.length === 0)
     return { value: "0 Kč", detail: null };
   const hasForeign = amounts.some((a) => a.currency !== "CZK");
-  if (hasForeign && totalCzk !== null && totalCzk !== undefined) {
+  if (hasForeign && totalCzk != null) {
     return {
       value: formatCurrency(totalCzk, "CZK"),
       detail: formatMultiCurrency(amounts),