Enables currency selection for expenses

Allows users to select a currency when adding an expense.

Fetches available currencies from the backend and displays them
in a dropdown menu, defaulting to EUR if the fetch fails.
The expense submission process now includes the selected currency.

src/modules/expenses/services/ExpensesAPI.ts

@@ -207,4 +207,25 @@ export class ExpensesAPI extends BaseService {
       throw error
     }
   }
+
+  /**
+   * Get available currencies from LNbits instance
+   */
+  async getCurrencies(): Promise<string[]> {
+    try {
+      const response = await fetch(`${this.baseUrl}/api/v1/currencies`, {
+        method: 'GET',
+        signal: AbortSignal.timeout(this.config?.apiConfig?.timeout || 30000)
+      })
+
+      if (!response.ok) {
+        throw new Error(`Failed to fetch currencies: ${response.statusText}`)
+      }
+
+      return await response.json()
+    } catch (error) {
+      console.error('[ExpensesAPI] Error fetching currencies:', error)
+      throw error
+    }
+  }
 }