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);

src/models/Payment.ts

@@ -4,8 +4,10 @@ export interface IPayment {
   _id?: string;
   title: string;
   description?: string;
-  amount: number;
-  currency: string;
+  amount: number; // Always in CHF (converted if necessary)
+  currency: string; // Currency code (CHF if no conversion, foreign currency if converted)
+  originalAmount?: number; // Amount in foreign currency (only if currency != CHF)
+  exchangeRate?: number; // Exchange rate used for conversion (only if currency != CHF)
   paidBy: string; // username/nickname of the person who paid
   date: Date;
   image?: string; // path to uploaded image
@@ -36,7 +38,17 @@ const PaymentSchema = new mongoose.Schema(
       type: String, 
       required: true, 
       default: 'CHF',
-      enum: ['CHF'] // For now only CHF as requested
+      uppercase: true
+    },
+    originalAmount: {
+      type: Number,
+      required: false,
+      min: 0
+    },
+    exchangeRate: {
+      type: Number,
+      required: false,
+      min: 0
     },
     paidBy: { 
       type: String, 

src/models/RecurringPayment.ts

@@ -4,16 +4,16 @@ export interface IRecurringPayment {
   _id?: string;
   title: string;
   description?: string;
-  amount: number;
-  currency: string;
+  amount: number; // Amount in the original currency
+  currency: string; // Original currency code
   paidBy: string; // username/nickname of the person who paid
   category: 'groceries' | 'shopping' | 'travel' | 'restaurant' | 'utilities' | 'fun' | 'settlement';
   splitMethod: 'equal' | 'full' | 'proportional' | 'personal_equal';
   splits: Array<{
     username: string;
-    amount?: number;
+    amount?: number; // Amount in original currency
     proportion?: number;
-    personalAmount?: number;
+    personalAmount?: number; // Amount in original currency
   }>;
   frequency: 'daily' | 'weekly' | 'monthly' | 'custom';
   cronExpression?: string; // For custom frequencies using cron syntax
@@ -47,7 +47,7 @@ const RecurringPaymentSchema = new mongoose.Schema(
       type: String,
       required: true,
       default: 'CHF',
-      enum: ['CHF']
+      uppercase: true
     },
     paidBy: {
       type: String,