feat: [IOBP-1898] Add show merchants button when webview url present (#7323)

## Short description
This PR introduces a new "Show merchants" secondary action to the IDPay
initiative details screen, provided the `webViewUrl` is included in the
response, allowing users to view participating merchants that redirect
to an external link specified in the backend response.

## List of changes proposed in this pull request
- Added a "Show merchants" (`secondaryCta`) button to the initiative
details footer, which opens the initiative's `webViewUrl` in the browser
if available; otherwise, it displays an error toast.

## How to test
With the dev-server started, verify that a bonus of type `DISCOUNT` with
the `webViewUrl` property filled shows a secondary CTA that redirects to
the provided URL as an external link, opening the device's browser.

## Preview


https://github.com/user-attachments/assets/84c79f6b-ca21-46d0-a9d4-196f2bd253c9

Author: Hantex9

locales/en/index.json

@@ -5648,7 +5648,8 @@
             "body": "If you forget the code, reset it under Settings > Security.",
             "cta": "Ok, chiudi"
           }
-        }
+        },
+        "secondaryCta": "Show merchants"
       },
       "beneficiaryDetails": {
         "infobox": {

locales/it/index.json

@@ -5629,6 +5629,7 @@
           "generateBarcode": "Genera un codice sconto"
         },
         "authorizeButton": "Usa il bonus",
+        "secondaryCta": "Mostra esercenti",
         "addExpenseButton": "Aggiungi spesa",
         "IDPayCode": {
           "banner": {

ts/features/idpay/details/screens/IdPayInitiativeDetailsScreen.tsx

@@ -3,6 +3,7 @@ import {
   ContentWrapper,
   H6,
   IOButton,
+  IOToast,
   Pictogram,
   VSpacer
 } from "@pagopa/io-app-design-system";
@@ -13,7 +14,7 @@ import { sequenceS } from "fp-ts/lib/Apply";
 import * as O from "fp-ts/lib/Option";
 import { pipe } from "fp-ts/lib/function";
 import { useCallback } from "react";
-import { View } from "react-native";
+import { Linking, View } from "react-native";
 import Animated, { LinearTransition } from "react-native-reanimated";
 import I18n from "i18next";
 import { ServiceId } from "../../../../../definitions/backend/ServiceId";
@@ -315,6 +316,14 @@ const IdPayInitiativeDetailsScreenComponent = () => {
       )
     );
 
+  const handleOnShowMerchants = () => {
+    if (!initiative.webViewUrl) {
+      IOToast.error(I18n.t("global.genericError"));
+      return;
+    }
+    void Linking.openURL(initiative.webViewUrl);
+  };
+
   const getInitiativeFooterProps = (
     rewardType?: InitiativeRewardTypeEnum
   ): IOScrollViewActions | undefined => {
@@ -326,12 +335,24 @@ const IdPayInitiativeDetailsScreenComponent = () => {
         ) {
           return;
         }
+        const useBonusButton = {
+          label: I18n.t("idpay.initiative.discountDetails.authorizeButton"),
+          onPress: discountBottomSheet.present
+        };
+        const showMerchantsButton = {
+          label: I18n.t("idpay.initiative.discountDetails.secondaryCta"),
+          onPress: handleOnShowMerchants
+        };
+        if (!initiative.webViewUrl) {
+          return {
+            type: "SingleButton",
+            primary: useBonusButton
+          };
+        }
         return {
-          type: "SingleButton",
-          primary: {
-            label: I18n.t("idpay.initiative.discountDetails.authorizeButton"),
-            onPress: discountBottomSheet.present
-          }
+          type: "TwoButtons",
+          primary: useBonusButton,
+          secondary: showMerchantsButton
         };
       }
       case InitiativeRewardTypeEnum.EXPENSE: