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/base.css

@@ -412,4 +412,9 @@ img {
     transition-duration: 0.01ms !important;
     scroll-behavior: auto !important;
   }
+
+  .admin-spinner {
+    animation-duration: 0.8s !important;
+    animation-iteration-count: infinite !important;
+  }
 }