Add currency settings, Settings service, and dialog to select fiat currency. Add support for non Official currencies like DOGE when using rates.

src/tui/screens/invitations/InvitationScreen.tsx

@@ -113,7 +113,7 @@ export function InvitationScreen(): React.ReactElement {
 
   const invitations = useInvitations();
   const { currencyCode, formattedFiatPerBchRate, formatSatoshisToFiat } =
-    useSatoshisConversion('USD');
+    useSatoshisConversion();
 
   // ── UI state ─────────────────────────────────────────────────────────────
   const [selectedIndex, setSelectedIndex] = useState(0);