Fetches and sets default currency for expenses

Ensures the expense form defaults to the LNbits instance's configured currency.

This change retrieves the default currency from the LNbits API and sets it as the initial value in the expense form. If no default is configured, it falls back to the first available currency or EUR.

src/modules/expenses/services/ExpensesAPI.ts

@@ -228,4 +228,26 @@ export class ExpensesAPI extends BaseService {
       throw error
     }
   }
+
+  /**
+   * Get default currency from LNbits instance
+   */
+  async getDefaultCurrency(): Promise<string | null> {
+    try {
+      const response = await fetch(`${this.baseUrl}/api/v1/default-currency`, {
+        method: 'GET',
+        signal: AbortSignal.timeout(this.config?.apiConfig?.timeout || 30000)
+      })
+
+      if (!response.ok) {
+        throw new Error(`Failed to fetch default currency: ${response.statusText}`)
+      }
+
+      const data = await response.json()
+      return data.default_currency
+    } catch (error) {
+      console.error('[ExpensesAPI] Error fetching default currency:', error)
+      throw error
+    }
+  }
 }