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/models/ExchangeRate.ts

@@ -0,0 +1,47 @@
+import mongoose from 'mongoose';
+
+export interface IExchangeRate {
+  _id?: string;
+  fromCurrency: string; // e.g., "USD"
+  toCurrency: string; // Always "CHF" for our use case
+  rate: number;
+  date: string; // Date in YYYY-MM-DD format
+  createdAt?: Date;
+  updatedAt?: Date;
+}
+
+const ExchangeRateSchema = new mongoose.Schema(
+  {
+    fromCurrency: {
+      type: String,
+      required: true,
+      uppercase: true,
+      trim: true
+    },
+    toCurrency: {
+      type: String,
+      required: true,
+      uppercase: true,
+      trim: true,
+      default: 'CHF'
+    },
+    rate: {
+      type: Number,
+      required: true,
+      min: 0
+    },
+    date: {
+      type: String,
+      required: true,
+      match: /^\d{4}-\d{2}-\d{2}$/
+    }
+  },
+  {
+    timestamps: true
+  }
+);
+
+// Create compound index for efficient lookups
+ExchangeRateSchema.index({ fromCurrency: 1, toCurrency: 1, date: 1 }, { unique: true });
+
+export const ExchangeRate = mongoose.model<IExchangeRate>("ExchangeRate", ExchangeRateSchema);