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/exchange-rates.ts

@@ -0,0 +1,65 @@
+/**
+ * Czech National Bank (ČNB) exchange rate service.
+ * Fetches daily rates and caches them.
+ * API: https://api.cnb.cz/cnbapi/exrates/daily
+ */
+
+interface CnbRate {
+  currencyCode: string;
+  rate: number;
+  amount: number;
+}
+
+const rateCache: Record<string, Record<string, number>> = {};
+
+async function fetchRatesForDate(
+  date?: string,
+): Promise<Record<string, number>> {
+  const key = date || "today";
+  if (rateCache[key]) return rateCache[key];
+
+  try {
+    let url = "https://api.cnb.cz/cnbapi/exrates/daily?lang=EN";
+    if (date) url += `&date=${date}`;
+
+    const response = await fetch(url);
+    if (!response.ok) throw new Error(`CNB API: ${response.status}`);
+
+    const data = (await response.json()) as { rates: CnbRate[] };
+    const rates: Record<string, number> = { CZK: 1 };
+
+    for (const r of data.rates) {
+      rates[r.currencyCode] = r.rate / r.amount;
+    }
+
+    rateCache[key] = rates;
+    return rates;
+  } catch (err) {
+    console.error("Failed to fetch CNB exchange rates:", err);
+    if (rateCache["today"]) return rateCache["today"];
+    return { CZK: 1, EUR: 25, USD: 22, GBP: 28 };
+  }
+}
+
+/** Convert an amount from a given currency to CZK using CNB rates */
+export async function toCzk(
+  amount: number,
+  currency: string,
+  date?: string,
+): Promise<number> {
+  if (currency === "CZK") return amount;
+  const rates = await fetchRatesForDate(date);
+  const rate = rates[currency];
+  if (!rate) return amount;
+  return Math.round(amount * rate * 100) / 100;
+}
+
+/** Get CNB rate for a currency (CZK per 1 unit), optionally for a specific date */
+export async function getRate(
+  currency: string,
+  date?: string,
+): Promise<number> {
+  if (currency === "CZK") return 1;
+  const rates = await fetchRatesForDate(date);
+  return rates[currency] || 1;
+}