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/routes/api/cospend/exchange-rates/+server.ts

@@ -0,0 +1,115 @@
+import type { RequestHandler } from '@sveltejs/kit';
+import { ExchangeRate } from '../../../../models/ExchangeRate';
+import { dbConnect } from '../../../../utils/db';
+import { error, json } from '@sveltejs/kit';
+
+export const GET: RequestHandler = async ({ locals, url }) => {
+  const auth = await locals.auth();
+  if (!auth || !auth.user?.nickname) {
+    throw error(401, 'Not logged in');
+  }
+
+  const fromCurrency = url.searchParams.get('from')?.toUpperCase();
+  const date = url.searchParams.get('date');
+  const action = url.searchParams.get('action') || 'rate';
+
+  if (action === 'currencies') {
+    return await getSupportedCurrencies();
+  }
+
+  if (!fromCurrency || !date) {
+    throw error(400, 'Missing required parameters: from and date');
+  }
+
+  if (!isValidCurrencyCode(fromCurrency)) {
+    throw error(400, 'Invalid currency code');
+  }
+
+  try {
+    const rate = await getExchangeRate(fromCurrency, date);
+    return json({ rate, fromCurrency, toCurrency: 'CHF', date });
+  } catch (e) {
+    console.error('Error getting exchange rate:', e);
+    throw error(500, 'Failed to get exchange rate');
+  }
+};
+
+async function getExchangeRate(fromCurrency: string, date: string): Promise<number> {
+  if (fromCurrency === 'CHF') {
+    return 1;
+  }
+
+  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 {
+    // Connection will be reused
+  }
+}
+
+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;
+}
+
+async function getSupportedCurrencies() {
+  try {
+    const response = await fetch('https://api.frankfurter.app/currencies');
+    
+    if (!response.ok) {
+      throw new Error(`API request failed: ${response.status}`);
+    }
+
+    const data = await response.json();
+    const currencies = Object.keys(data);
+    
+    return json({ currencies });
+  } catch (e) {
+    console.error('Error fetching supported currencies:', e);
+    // Return common currencies as fallback
+    const fallbackCurrencies = ['EUR', 'USD', 'GBP', 'JPY', 'CAD', 'AUD', 'SEK', 'NOK', 'DKK'];
+    return json({ currencies: fallbackCurrencies });
+  }
+}
+
+function isValidCurrencyCode(currency: string): boolean {
+  return /^[A-Z]{3}$/.test(currency);
+}