feat: add multi-currency support to cospend payments

- Add ExchangeRate model for currency conversion tracking
- Implement currency utility functions for formatting and conversion
- Add exchange rates API endpoint with caching and fallback rates
- Update Payment and RecurringPayment models to support multiple currencies
- Enhanced payment forms with currency selection and conversion display
- Update split method selector with better currency handling
- Add currency-aware payment display and balance calculations
- Support for EUR, USD, GBP, and CHF with automatic exchange rate fetching

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/lib/utils/currency.ts

@@ -0,0 +1,94 @@
+import { ExchangeRate } from '../../models/ExchangeRate';
+import { dbConnect, dbDisconnect } from '../../utils/db';
+
+/**
+ * Convert amount from foreign currency to CHF using direct database/API access
+ */
+export async function convertToCHF(
+  amount: number,
+  fromCurrency: string,
+  date: string,
+  fetch?: typeof globalThis.fetch
+): Promise<{
+  convertedAmount: number;
+  exchangeRate: number;
+}> {
+  if (fromCurrency.toUpperCase() === 'CHF') {
+    return {
+      convertedAmount: amount,
+      exchangeRate: 1
+    };
+  }
+
+  const rate = await getExchangeRate(fromCurrency.toUpperCase(), date);
+
+  return {
+    convertedAmount: amount * rate,
+    exchangeRate: rate
+  };
+}
+
+/**
+ * Get exchange rate from database cache or fetch from API
+ */
+async function getExchangeRate(fromCurrency: string, date: string): Promise<number> {
+  const dateStr = date.split('T')[0]; // Extract YYYY-MM-DD
+  
+  await dbConnect();
+  
+  try {
+    // Try cache first
+    const cachedRate = await ExchangeRate.findOne({
+      fromCurrency,
+      toCurrency: 'CHF',
+      date: dateStr
+    });
+
+    if (cachedRate) {
+      return cachedRate.rate;
+    }
+
+    // Fetch from API
+    const rate = await fetchFromFrankfurterAPI(fromCurrency, dateStr);
+    
+    // Cache the result
+    await ExchangeRate.create({
+      fromCurrency,
+      toCurrency: 'CHF',
+      rate,
+      date: dateStr
+    });
+
+    return rate;
+  } finally {
+    await dbDisconnect();
+  }
+}
+
+/**
+ * Fetch exchange rate from Frankfurter API
+ */
+async function fetchFromFrankfurterAPI(fromCurrency: string, date: string): Promise<number> {
+  const url = `https://api.frankfurter.app/${date}?from=${fromCurrency}&to=CHF`;
+  
+  const response = await fetch(url);
+  
+  if (!response.ok) {
+    throw new Error(`Frankfurter API request failed: ${response.status}`);
+  }
+
+  const data = await response.json();
+  
+  if (!data.rates || !data.rates.CHF) {
+    throw new Error(`No exchange rate found for ${fromCurrency} to CHF on ${date}`);
+  }
+
+  return data.rates.CHF;
+}
+
+/**
+ * Validate currency code (3-letter ISO code)
+ */
+export function isValidCurrencyCode(currency: string): boolean {
+  return /^[A-Z]{3}$/.test(currency.toUpperCase());
+}