feat: restore Runiczny Pancerz with Mana; NBP rates module with latest-rate fallback

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/logic/nbp.ts

@@ -0,0 +1,53 @@
+/**
+ * NBP (Narodowy Bank Polski) table A mid rates for the money converter.
+ * Campaign year → first published rate on/after 1 June of that year; a year in the
+ * future (or no table found / request failed) → latest published rate.
+ */
+
+import { TRADE } from './rules';
+
+export interface NbpRate {
+  rate: number;
+  /** Effective date (YYYY-MM-DD) of the table the rate comes from. */
+  date: string;
+}
+
+const NBP_FIRST_YEAR = 2002;
+const NBP_TIMEOUT_MS = 4000;
+
+/** Offline fallbacks when NBP is unreachable. */
+export const FALLBACK_RATES: Record<string, number> = { eur: TRADE.eurRate, usd: 4.0 };
+
+type FetchLike = (url: string, init?: { signal?: AbortSignal }) => Promise<{ ok: boolean; json(): Promise<any> }>;
+
+async function getRates(url: string, fetchImpl: FetchLike): Promise<NbpRate | null> {
+  const ctrl = new AbortController();
+  const timer = setTimeout(() => ctrl.abort(), NBP_TIMEOUT_MS);
+  try {
+    const res = await fetchImpl(url, { signal: ctrl.signal });
+    if (!res.ok) return null;
+    const first = (await res.json())?.rates?.[0];
+    return typeof first?.mid === 'number' ? { rate: first.mid, date: String(first.effectiveDate ?? '') } : null;
+  } catch {
+    return null;
+  } finally {
+    clearTimeout(timer);
+  }
+}
+
+/** Fetch the rate for `currency` (ISO code, any case) valid for the given campaign year. */
+export async function fetchNbpRate(
+  currency: string,
+  year: number,
+  now: Date = new Date(),
+  fetchImpl: FetchLike = (u, i) => fetch(u, i),
+): Promise<NbpRate | null> {
+  const code = currency.toLowerCase();
+  const base = `https://api.nbp.pl/api/exchangerates/rates/a/${code}`;
+  if (year >= NBP_FIRST_YEAR && year <= now.getFullYear()) {
+    // A two-week window always contains a working day; take the first table in it.
+    const hit = await getRates(`${base}/${year}-06-01/${year}-06-14/?format=json`, fetchImpl);
+    if (hit) return hit;
+  }
+  return getRates(`${base}/?format=json`, fetchImpl);
+}