feat(04-01): add locale parameter to formatCurrency, default 'en'

- Replace hardcoded 'de-DE' with optional locale parameter defaulting to 'en'
- Defensive locale || 'en' guard prevents RangeError on empty string
- All existing call sites get English formatting (FIX-01)
- Third arg enables locale-aware chart tooltips in follow-on plans

frontend/src/lib/format.ts

@@ -1,5 +1,9 @@
-export function formatCurrency(amount: number, currency: string = 'EUR'): string {
-  return new Intl.NumberFormat('de-DE', {
+export function formatCurrency(
+  amount: number,
+  currency: string = 'EUR',
+  locale: string = 'en'
+): string {
+  return new Intl.NumberFormat(locale || 'en', {
     style: 'currency',
     currency,
   }).format(amount)