Fetches and applies real-time exchange rate

Fetches the current BTC/EUR exchange rate from the CoinGecko API and uses it to calculate fiat values.

This ensures more accurate currency conversions, especially for expenses and payments. A fallback to a default rate is included in case the API is unavailable.

static/js/index.js

@@ -19,6 +19,7 @@ window.app = Vue.createApp({
       isSuperUser: false,
       castleWalletConfigured: false,
       userWalletConfigured: false,
+      currentExchangeRate: null, // BTC/EUR rate (sats per EUR)
       expenseDialog: {
         show: false,
         description: '',
@@ -97,7 +98,7 @@ window.app = Vue.createApp({
         paymentHash: null,
         checkWalletKey: null,
         pollIntervalId: null,
-        exchangeRate: 3571.43, // sats per EUR (TODO: fetch from API)
+        exchangeRate: null, // Will be set from currentExchangeRate
         originalCurrency: 'BTC' // Track original receivable currency
       },
       payUserDialog: {
@@ -113,7 +114,7 @@ window.app = Vue.createApp({
         reference: '',
         loading: false,
         paymentSuccess: false,
-        exchangeRate: 3571.43,
+        exchangeRate: null, // Will be set from currentExchangeRate
         originalCurrency: 'BTC'
       }
     }
@@ -1012,7 +1013,7 @@ window.app = Vue.createApp({
         paymentHash: null,
         checkWalletKey: null,
         pollIntervalId: null,
-        exchangeRate: fiatAmount > 0 ? Math.abs(userBalance.balance) / fiatAmount : 3571.43, // Calculate rate from actual amounts
+        exchangeRate: fiatAmount > 0 ? Math.abs(userBalance.balance) / fiatAmount : (this.currentExchangeRate || 3571.43), // Calculate rate from actual amounts or use current rate
         originalCurrency: fiatCurrency || 'BTC'
       }
     },
@@ -1213,7 +1214,7 @@ window.app = Vue.createApp({
         reference: '',
         loading: false,
         paymentSuccess: false,
-        exchangeRate: fiatAmount > 0 ? userBalance.balance / fiatAmount : 3571.43,
+        exchangeRate: fiatAmount > 0 ? userBalance.balance / fiatAmount : (this.currentExchangeRate || 3571.43),
         originalCurrency: fiatCurrency || 'BTC'
       }
     },
@@ -1357,6 +1358,27 @@ window.app = Vue.createApp({
         return null
       }
     },
+    async loadExchangeRate() {
+      try {
+        // Fetch current BTC/EUR rate from CoinGecko API
+        const response = await fetch('https://api.coingecko.com/api/v3/simple/price?ids=bitcoin&vs_currencies=eur')
+        const data = await response.json()
+
+        if (data && data.bitcoin && data.bitcoin.eur) {
+          // Convert BTC/EUR to sats/EUR (1 BTC = 100,000,000 sats)
+          const btcPerEur = 1 / data.bitcoin.eur
+          this.currentExchangeRate = Math.round(btcPerEur * 100000000)
+          console.log('Exchange rate loaded:', this.currentExchangeRate, 'sats per EUR')
+        } else {
+          throw new Error('Invalid response from exchange rate API')
+        }
+      } catch (error) {
+        console.error('Error loading exchange rate:', error)
+        // Fallback to a reasonable default if API fails
+        this.currentExchangeRate = 3571.43
+        console.log('Using fallback exchange rate:', this.currentExchangeRate)
+      }
+    },
     formatSats(amount) {
       return new Intl.NumberFormat().format(amount)
     },
@@ -1425,6 +1447,7 @@ window.app = Vue.createApp({
     // Load settings first to determine if user is super user
     await this.loadSettings()
     await this.loadUserWallet()
+    await this.loadExchangeRate()
     await this.loadBalance()
     await this.loadTransactions()
     await this.loadAccounts()