feat: Disable 'Create Invoice' button until an amount is entered

Fixes : getAlby#2802

The button is now disabled until the user enters a value in the 'Amount' field. This ensures that the user cannot create an invoice without specifying the amount first.

This change helps to prevent invalid or incomplete invoice creation.

src/app/screens/ReceiveInvoice/index.tsx

@@ -264,7 +264,11 @@ function ReceiveInvoice() {
                   fullWidth
                   primary
                   loading={loadingInvoice}
-                  disabled={loadingInvoice}
+                  disabled={
+                    loadingInvoice ||
+                    !formData.amount ||
+                    formData.amount === "0"
+                  }
                 />
               </Container>
             </fieldset>