refactor: extract shared formatEur and getTradeStatusDisplay utilities

- Create frontend/app/utils/exchange.ts with shared formatting functions
- Update exchange/page.tsx to use shared formatEur
- Update trades/page.tsx to use shared formatEur and getTradeStatusDisplay
- Update admin/trades/page.tsx to use shared formatEur and getTradeStatusDisplay
- SatsDisplay was already extracted (confirmed it's in components/SatsDisplay.tsx)

frontend/app/exchange/page.tsx

@@ -10,6 +10,7 @@ import { SatsDisplay } from "../components/SatsDisplay";
 import { useRequireAuth } from "../hooks/useRequireAuth";
 import { components } from "../generated/api";
 import { formatDate, formatTime, getDateRange } from "../utils/date";
+import { formatEur } from "../utils/exchange";
 import { layoutStyles, typographyStyles, bannerStyles, buttonStyles } from "../styles/shared";
 
 type ExchangePriceResponse = components["schemas"]["ExchangePriceResponse"];
@@ -24,13 +25,6 @@ const MAX_ADVANCE_DAYS = 30;
 type Direction = "buy" | "sell";
 type WizardStep = "details" | "booking";
 
-/**
- * Format EUR amount from cents to display string
- */
-function formatEur(cents: number): string {
-  return `€${(cents / 100).toLocaleString("de-DE")}`;
-}
-
 /**
  * Format price for display
  */