fix(share): attach captured card image when sharing to Instagram Stories

Both share modals opened the instagram-stories:// scheme without ever
placing the captured PNG on the pasteboard, so Instagram opened an empty
story. Replace the manual scheme launch with react-native-share
shareSingle(INSTAGRAM_STORIES), which writes the image to UIPasteboard
under com.instagram.sharedSticker.backgroundImage on iOS and uses the
ADD_TO_STORY intent on Android. Gate the flow on the required Facebook
App ID (EXPO_PUBLIC_FACEBOOK_APP_ID) and Instagram being installed, with
fallback to the system share sheet, and remove the dead base64 reads.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/mobile/app.json

@@ -298,7 +298,19 @@
         }
       ],
       "./plugins/withWidgetExported.js",
-      "./plugins/withNativeWidget.js"
+      "./plugins/withNativeWidget.js",
+      [
+        "react-native-share",
+        {
+          "ios": [
+            "instagram",
+            "instagram-stories"
+          ],
+          "android": [
+            "com.instagram.android"
+          ]
+        }
+      ]
     ],
     "experiments": {
       "typedRoutes": true