Add oracle rates

src/tui/hooks/useSatoshisConversion.tsx

@@ -0,0 +1,42 @@
+import { useCallback, useMemo } from 'react';
+import { useAppContext } from './useAppContext.js';
+import { useBchToFiatRate } from './useRates.js';
+
+/**
+ * Reactive BCH satoshis -> fiat conversion helpers for TUI screens.
+ *
+ * This hook subscribes to rate updates through `useBchToFiatRate`, so any
+ * component using it will re-render automatically when the selected pair
+ * receives a new quote.
+ */
+export function useSatoshisConversion(targetCurrency: string = 'USD') {
+  const { appService } = useAppContext();
+  const currencyCode = useMemo(() => targetCurrency.toUpperCase(), [targetCurrency]);
+  const fiatPerBchRate = useBchToFiatRate(currencyCode);
+
+  const formattedFiatPerBchRate = useMemo(() => {
+    if (!appService || fiatPerBchRate === null) {
+      return null;
+    }
+
+    return appService.rates.formatCurrency(fiatPerBchRate, currencyCode);
+  }, [appService, fiatPerBchRate, currencyCode]);
+
+  const formatSatoshisToFiat = useCallback(
+    (satoshis: bigint): string | null => {
+      if (!appService || fiatPerBchRate === null) {
+        return null;
+      }
+
+      return appService.rates.formatBchToFiat(satoshis, currencyCode);
+    },
+    [appService, fiatPerBchRate, currencyCode],
+  );
+
+  return {
+    currencyCode,
+    fiatPerBchRate,
+    formattedFiatPerBchRate,
+    formatSatoshisToFiat,
+  } as const;
+}